External cavity laser based wavelength division multiplexing superchannel transceivers

ABSTRACT

A technique relates to a superchannel. Laser cavities include a first laser cavity, a next laser cavity, through a last laser cavity. Modulators include a first modulator, a next modulator, through a last modulator, each having a direct input, an add port, and an output. A concatenated arrangement of the laser cavities is configured to form the superchannel, which includes the last laser cavity coupled to the direct input of the last modulator, and the output of the last modulator coupled to the add port of the next modulator. The arrangement includes the next laser cavity coupled to direct input of the next modulator, and the output of the next modulator coupled to add port of first modulator, along with the first laser cavity coupled to direct input of the first modulator, and the output of first modulator coupled to input of a multiplexer, thus forming the superchannel into multiplexer.

BACKGROUND

The present invention relates to an external cavity laser on silicon,and more specifically, to silicon external cavity laser basedsuperchannel transceivers.

An optical cavity or optical resonator is an arrangement of mirrors thatforms a standing wave cavity resonator for light waves. Optical cavitiesare a major component of lasers, surrounding the gain medium andproviding feedback of the laser light. They are also used in opticalparametric oscillators and some interferometers. Light confined in thecavity reflects multiple times, producing standing waves for certainresonance frequencies. The standing wave patterns produced are calledmodes. Longitudinal modes differ only in frequency while transversemodes differ for different frequencies and have different intensitypatterns across the cross section of the beam.

Different resonator types are distinguished by the focal lengths of thetwo mirrors and the distance between them. Flat mirrors are not oftenused because of the difficulty of aligning them to the needed precision.The geometry (resonator type) must be chosen so that the beam remainsstable, which means that the size of the beam does not continually growwith multiple reflections. Resonator types are also designed to meetother criteria such as minimum beam waist or having no focal pointinside the cavity. Optical cavities are designed to have a large Qfactor, which means that the light beam will reflect a very large numberof times with little attenuation. Therefore, the frequency line width ofthe beam is very small compared to the frequency of the laser.

Light confined in a resonator will reflect multiple times from themirrors, and due to the effects of interference, only certain patternsand frequencies of radiation will be sustained by the resonator, withthe others being suppressed by destructive interference. In general,radiation patterns which are reproduced on every round-trip of the lightthrough the resonator are the most stable, and these are the eigenmodes,known as the modes, of the resonator.

Resonator modes can be divided into two types: longitudinal modes, whichdiffer in frequency from each other; and transverse modes, which maydiffer in both frequency and the intensity pattern of the light. Thebasic or fundamental transverse mode of a resonator is a Gaussian beam.

The most common types of optical cavities consist of two facing plane(flat) or spherical mirrors. The simplest of these is the plane-parallelor Fabry-Pérot cavity, consisting of two opposing flat mirrors.Plane-parallel resonators are therefore commonly used in microchiplasers, microcavity lasers, and semiconductor lasers. In these cases,rather than using separate mirrors, a reflective optical coating may bedirectly applied to the laser medium itself.

SUMMARY

According to one embodiment, a semiconductor chip configured to form asuperchannel is provided. The semiconductor chip includes a plurality oflaser cavities including a first laser cavity, a next laser cavity,through a last laser cavity, a wavelength division multiplexing (WDM)multiplexer, and a plurality of modulators including a first modulator,a next modulator, through a last modulator. Each of the plurality ofmodulators has a direct input, an add port, and an output. Aconcatenated arrangement of the plurality of laser cavities isconfigured to form the superchannel. The concatenated arrangementincludes the last laser cavity coupled to the direct input of the lastmodulator, and the output of the last modulator coupled to the add portof the next modulator. Also, the concatenated arrangement includes thenext laser cavity coupled to the direct input of the next modulator, andthe output of the next modulator coupled to the add port of the firstmodulator. Further, the concatenated arrangement includes the firstlaser cavity coupled to the direct input of the first modulator, and theoutput of the first modulator coupled to one input of the WDMmultiplexer, thus forming the superchannel being input into the oneinput of the WDM multiplexer.

According to one embodiment, a method of creating a superchannel on asemiconductor chip is provided. The method includes forming a pluralityof laser cavities including a first laser cavity, a next laser cavity,through a last laser cavity and providing a plurality of modulatorsincluding a first modulator, a next modulator, through a last modulator.Each of the plurality of modulators has a direct input, an add port, andan output. Also, the method includes configuring a concatenatedarrangement of the plurality of laser cavities to form a superchannel.The concatenated arrangement includes the last laser cavity coupled tothe direct input of the last modulator, and the output of the lastmodulator coupled to the add port of the next modulator. Also, theconcatenated arrangement includes the next laser cavity coupled to thedirect input of the next modulator, and the output of the next modulatorcoupled to the add port of the first modulator. Further, theconcatenated arrangement includes the first laser cavity coupled to thedirect input of the first modulator, and the output of the firstmodulator coupled to one input of a wavelength division multiplexing(WDM) multiplexer, thus forming the superchannel being input into theone input of the WDM multiplexer.

According to one embodiment, a semiconductor chip configured as areceiver to receive a superchannel is provided. The semiconductor chipincludes a polarization splitter rotator configured to receive and splitreceived light of the superchannel, wavelength division multiplexing(WDM) demultiplexers configured to demultiplex the received light, andcounter propagating drop filters configured to capture the receivedlight at a particular target wavelength and generate an electricalsignal. Each of the counter propagating drop filters is coupled to anelectrical receiver, and the electrical receiver receives the electricalsignal corresponding to the particular target wavelength.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a single frequency cavity diagram of a laser on asilicon chip according to an embodiment;

FIG. 2 illustrates a single frequency cavity diagram of a laser on asilicon chip according to an embodiment;

FIG. 3A illustrates a graph showing how the phase changes and passivephase compensation temperature changes correspondingly, according to anembodiment;

FIG. 3B illustrates a schematic of the passive intra-cavitytransmission-mode optical phase compensation element with detailsaccording to and embodiment;

FIG. 4 illustrates an example intra-cavity transmission-mode opticalband-pass filter according to an embodiment;

FIG. 5A illustrates an implementation of an active intra-cavitytransmission-mode thermo-optic optical phase tuner element as abroadband thermo-optic tuner according to an embodiment;

FIG. 5B illustrates another implementation of the active intra-cavitytransmission-mode thermo-optic optical phase tuner element as narrowbandthermo-optic tuners according to an embodiment;

FIG. 6 illustrates an example output coupler band-reflect gratingoptical filter according to an embodiment;

FIG. 7 illustrates an example of a mode converter according to anembodiment;

FIG. 8 illustrates an example implementation of a power monitoraccording to an embodiment;

FIG. 9 illustrates a multi-frequency diagram of a laser on the siliconchip according to an embodiment;

FIG. 10 illustrates another multi-frequency diagram of a laser on thesilicon chip according to an embodiment;

FIG. 11 illustrates a method of configuring the semiconductor chipaccording to an embodiment;

FIG. 12 illustrates a diagram for integrated tunable lasers with acoarse wavelength division multiplexing (CWDM) multiplexer on asemiconductor chip according to an embodiment;

FIG. 13 illustrates graphs of laser wavelength tuning within the passband of wavelength division multiplexing multiplexers for a single laserand superchannel of multiple lasers according to an embodiment;

FIGS. 14A and 14B together illustrate a block diagram of aself-calibration routine according to an embodiment;

FIG. 15 illustrates a block diagram of normal operation control afterthe self-calibration routine according to an embodiment;

FIG. 16A is a graph illustrating an allowable target wavelength range atwhich to make/insert the target wavelength in order to pass through thepass band of the N×1 port CWDM multiplexer according to an embodiment;

FIG. 16B is a graph illustrating the allowable target wavelength rangeat which to make/insert the target wavelength according to anembodiment;

FIG. 17A is a graph illustrating an allowable target wavelength range atwhich to make/insert multiple target wavelengths (that form asuperchannel) in order to pass through the pass band of the N×1 portCWDM multiplexer according to an embodiment;

FIG. 17B is a graph illustrating the allowable target wavelength rangeat which to make/insert the multiple target wavelengths according to anembodiment;

FIGS. 18A and 18B together illustrate a method for configuring asemiconductor chip to have a self-calibrated laser according to anembodiment.

FIG. 19 illustrates a superchannel laser transmitter on a silicon chipaccording to an embodiment;

FIG. 20 illustrates an example implementation of a 2×2 port modulatoraccording to an embodiment;

FIG. 21 illustrates a superchannel laser receiver according to anembodiment; and

FIG. 22 is a method of creating one or more superchannels on asemiconductor chip according to an embodiment.

DETAILED DESCRIPTION

Although laser light is perhaps the purest form of light, it is not of asingle, pure frequency or wavelength. All lasers produce light over somenatural bandwidth or range of frequencies. A laser's bandwidth ofoperation is determined primarily by the gain medium from which thelaser is constructed and by the range of frequencies over which a lasermay operate (known as the gain bandwidth).

The second factor to determine a laser's emission frequencies is theoptical cavity (or resonant cavity) of the laser. In the simplest case,this consists of two plane (flat) mirrors facing each other, surroundingthe gain medium of the laser (again this arrangement is known as aFabry-Pérot cavity). Since light is a wave, when bouncing between themirrors of the cavity, the light will constructively and destructivelyinterfere with itself, leading to the formation of standing waves ormodes between the mirrors. These standing waves form a discrete set offrequencies, known as the longitudinal modes of the cavity. These modesare the only frequencies of light which are self-regenerating andallowed to oscillate by the resonant cavity, while all other frequenciesof light are suppressed by destructive interference. For a simpleplane-mirror cavity, the allowed modes are those for which theseparation distance of the mirrors L is an exact multiple of half thewavelength of the light λ, such that L=qλ/2, where q is an integer knownas the mode order.

In a simple laser, each of these modes oscillates independently, with nofixed relationship between each other, in essence like a set ofindependent lasers all emitting light at slightly different frequencies.The individual phase of the light waves in each mode is not fixed andmay vary randomly due to such things as thermal changes (i.e.,temperature) in materials of the laser. In lasers with only a fewoscillating modes, interference between the modes can cause beatingeffects in the laser output, leading to fluctuations in intensity. Inlasers with many thousands of modes, these interference effects tend toaverage to a near-constant output intensity.

Embodiments are configured to provide temperature insensitive (i.e.,thermal insensitive) optical laser cavities. According to embodiments,the temperature insensitive external cavity lasers on silicon providevarious benefits:

(1) Cost reduction is achievable by simplifying laser fabrication andeliminating operating wavelength tolerance as yield limitation. Lasingfrequency is set by silicon fabricated components that have a highfabrication precision and inherent tunability with no extra cost.

(2) Stabilization of lasing frequency is achievable by active or passivemeans in the silicon fabricated section at any desired temperaturethroughout operation range.

(3) Relative intensity noise (RIN) may be reduced (performanceimprovement) by the cavity length increase and high-extinctionintra-cavity optical filter.

(4) Narrowband filter used in the passive cavity may enable siliconon-chip isolator through time gating modulators at transceiver bitrate.

(5) The III-V chip is identical to plan-of-record distributed feedback(DFB) lasers except that the grating fabrication step is omitted by thelaser vendor.

Now turning to the figures, FIG. 1 illustrates a single frequency cavitydiagram of a laser on a silicon chip 100 according to an embodiment. Thesilicon chip 100 is a laser or laser system. Although silicon isdiscussed as an example chip and substrate material, it is understoodthat other semiconductor materials may be utilized including a germaniumwafer.

The silicon chip 100 has a III-V chip 10 mounted on the substrate 30(e.g., silicon wafer) of the silicon chip 100. The III-V chip 10 mayalso be referred to as a III-V die, a III-V semiconductor chip, and/anoptical gain chip/medium as understood by one skilled in the art. Thecombination of the III-V chip 10 mounted on the silicon substrate 30 ofthe silicon chip 100 may be referred to as a hybrid silicon laser. Thehybrid silicon laser is a semiconductor laser fabricated from bothsilicon and group III-V semiconductor materials. Group III and group Vare designations on the periodic table. The hybrid approach takesadvantage of the light-emitting properties of III-V semiconductormaterials combined with the process maturity of silicon to fabricateelectrically driven lasers on a silicon wafer that can be integratedwith other silicon photonic devices.

The III-V chip 10 may be a laser diode that is an electrically pumpedsemiconductor laser in which the active (gain) medium is formed by a p-njunction (p-type doped region and n-type doped region) of asemiconductor diode similar to that found in a light-emitting diode. Alaser diode is electrically a PIN diode (also referred to as a p-i-ndiode), which is a diode with a wide, undoped intrinsic (I)semiconductor region between a p-type (P) semiconductor and an n-type(N) semiconductor region. The p-type and n-type regions are typicallyheavily doped because they are used for ohmic contacts. The active(gain) region of the laser diode is in the intrinsic (I) region, and thecarriers (i.e., electrons and holes) are pumped into intrinsic (I)region from the N and P regions respectively. While initial diode laserresearch was conducted on simple P-N diodes, modern lasers use thedouble-heterostructure implementation, where the carriers and thephotons are confined in order to maximize their chances forrecombination and light generation. Unlike a regular diode used inelectronics, the goal for a laser diode is that all carriers recombinein the I region and thus produce light. Accordingly, laser diodes arefabricated using direct bandgap semiconductors. The laser diodeepitaxial structure is grown using one of the crystal growth techniques,usually starting from an N doped substrate, and growing the I dopedactive layer, followed by the P doped cladding, and a contact layer. Theactive layer most often consists of quantum wells, which provide lowerthreshold current and higher efficiency. A method of powering some laserdiode is the use of optical pumping. Optically pumped semiconductorlasers (OPSL) use the III-V semiconductor chip 10 as the gain medium,and use another laser (often another diode laser) as the pump source.One skilled in the art understands the use and operation of a laserusing a III-V semiconductor chip.

Referring back to FIG. 1, the III-V chip 10 has a high rear reflective(HR) coating facet 12 on one end and has antireflective (AR) coatingfacet 14 on the other end. The light increases in intensity in the gainregion (intrinsic region (I)) of the III-V chip 10.

The III-V chip 10 is attached/mounted to silicon chip 100 and alignedfor optical coupling by any flip-chip or wirebond mounting option knownto one skilled in the art. The III-V chip 10 (e.g., a hybrid siliconlaser) is an optical source that is fabricated from both silicon andgroup III-V semiconductor materials, where the group III-V semiconductormaterials may include, e.g., Indium (III) phosphide (V), gallium (III)arsenide (V), nitrogen (V), etc. A mode converter 16 is coupled to theIII-V chip 10. In one case, the mode converter 16 may be identical tothat required to couple a distributed feedback laser (DFB) with similarrequirements for low insertion loss and reflection as understood by oneskilled in the art. A mode converter 16 (also referred to as mode sizeconverter) includes optical devices which allow for efficient couplingbetween modes of different sizes. A mode (size) converter (or mode sizeadapter) is an optical device which can be used for expanding orcontracting a mode in the transverse spatial dimensions. For example, amode converter could expand the very tiny mode of the waveguide in alaser diode to a size which fits to the mode of an optical fiber.

The mode converter 16 couples the III-V chip 10 to the waveguide 20.Although shown schematically as entirely external to the III-V chip 10,it is understood that mode converting components 16 may also includecomponents, e.g. active or passive waveguide sections with differentdimensions that the primary gain waveguide section, fabricated on theIII-V chip 10. The waveguide 20 connects to various intra-cavity opticalelements 120, 130, and 140 on the silicon chip 100 as understood by oneskilled in the art. The order of the intra-cavity optical elements 120and 130 shown is arbitrary and is not meant to be limiting. It iscontemplated that either order of the intra-cavity optical elements 120and 130 is possible. The intra-cavity optical elements 120, 130, 140 maybe an external integrated photonic circuit 25 fabricated on the siliconsubstrate 30 of the silicon chip 100.

A laser cavity 24 is formed between the III-V gain chip 10 and theexternal integrated photonic circuit 25, specifically between the HRfacet 12 and the band-reflect grating 140. To provide a basis for thefollowing discussion, the magnitude of the dominant polarization of theelectric field, E or modal amplitude, in the laser resonator will bedescribed as a function of time, t, and longitudinal position, z. Thecoordinate system is defined such that the HR facet (element 12) of theIII-V chip (element 10) is z=0. The expression for the modal amplitudecan then be described by the functionE(ω,z,t,T)=A_(forward)(z)·e^(i(ωt-k(ω,z,T)z))+A_(reverse)(z)·e^(i(k(ω,z,T)z-ωt)).The real valued A_(forward)(z) value A_(reverse)(z) and functions definethe amplitude of the forward and backward propagating fields in thelaser cavity subject to the loss and gain from the intra-cavityelements. Remaining variables are defined as follows: ω is the angularfrequency of the optical mode of interest; T is the local temperature(i.e. rigorously T(z)); k(ω,z,T) is the wavevector of the optical modeof the given angular frequency, for the given longitudinal position andtemperature. For clarity, the effects of reflections from intra-cavityelements are neglected and details associated with the phase changeresulting from transmission through the intra-cavity filter areneglected. All intra-cavity elements are treated as waveguides witharbitrary k(z,ω,T) characteristics.

The laser cavity as defined then supports a continuum of longitudinaloptical modes ω₀, ω₁, . . . ω_(m) that are determined by the round-tripconstructive interference condition of the resonator. As is well knownwithin the field, this interference condition is satisfied when theaccumulated optical phase of the round-trip propagation, φ, equals aninteger multiple of 2π. Using the above conventions and defining theposition of effective reflection within the band-reflect grating 140 forthe given modal angular frequency and local temperature as z″(ω,T), theround trip phase is given by:

φ(ω,T)=2∫₀ ^(z″(ω,T)) k(ω,z,T)zdz

To simplify the analysis, the case of uniform, frequency independentmodal effective indices, n_(III-V)(T) and n_(Si)(T), will be consideredfor the III-V chip 10 and the silicon external cavity 25 respectively.The longitudinal coordinate for the interface between the III-V chip 10and the silicon external cavity 25 is then defined as z′ with thelengths of the two cavity halves as L_(III-V) and L_(Si)(T)respectively. The length of the silicon external cavity 25 is stillconsidered temperature dependent in this analysis due to z″(ω,T), butthe frequency dependence is ignored. The resulting round trip phase canthen be expressed simply by expanding the wavevector in terms of theeffective index, angular frequency and vacuum speed of light, c, as afunction of position:

${\phi \left( {\omega,T} \right)} = {2\frac{\omega}{c}\left( {{{n_{{III}\text{-}V}(T)}L_{{III}\text{-}V}} + {{n_{Si}(T)}{L_{Si}(T)}}} \right)}$

Enforcing the phase matching condition, the angular frequency ofoperating mode ω_(m) can then be expressed as:

${\omega_{m}(T)} = \frac{\left( {m + 1} \right)\pi \; c}{{{n_{{III}\text{-}V}(T)}L_{{III}\text{-}V}} + {{n_{Si}(T)}{L_{Si}(T)}}}$

Now, further details of the intra-cavity optical elements 120, 130, 140in the external integrated photonic circuit 25 are discussed below inthe context of the above influence on ω_(m)(T).

The intra-cavity optical element 120 is an intra-cavitytransmission-mode optical band-pass filter 120 with a full-widthhalf-maximum (FWHM) equal to or less than the free-spectral range of thecavity Fabry-Perot (F-P) resonances. The purpose of this filter is toprovide operating longitudinal mode selection through lossdiscrimination such that the filter resonant frequency, ω_(f), isactively tuned to be centered on the desired ω_(m) while providingsufficient round-trip cavity loss discrimination for adjacentlongitudinal modes ω_(m−1) and ω_(m+1) to prevent undesired modes fromreaching lasing threshold and provide a sufficient side mode suppressionratio. The narrow bandwidth of the intra-cavity filter results in areduction of laser output power proportional to the magnitude ofdifference, Δω=|ω_(f)−ω_(m)|. This enables the output power of the laserto be monitored as a feedback parameter for matching the intra-cavityfilter resonance frequency with the longitudinal operating mode throughthe active control of either mode.

The resonance frequency of the intra-cavity transmission-mode opticalband-pass filter 120 is then held constant throughout laser operation(except startup initialization or where intentionally modulated) as afunction of temperature through either an athermal design or activecontrol. Examples of athermal design for the band-pass filter includemodal thermo-optic coefficient compensation by varying waveguide widthsand lengths in a silicon/silicon dioxide interferometer or introducingnegative thermo-optic material cladding such as TiO₂ over a siliconnanowire ring resonator filter. Examples of active control includecontrolling integrated heater power based on a temperature sensorfeedback signal.

In the case of low free-spectral range filters such as ring resonators,the intra-cavity transmission-mode optical band-pass filter 120 mustalso be designed such that the free-spectral range is greater than halfof the FWHM reflection bandwidth of the band-reflect grating 140. Thiscondition ensures that other longitudinal mode orders of the band-passfilter do not provide alternate low round-trip loss longitudinal lasercavity operating modes.

The intra-cavity optical element 130 is an active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130. Theactive intra-cavity transmission-mode thermo-optic optical phase tunerelement 130 may include either a broad-band waveguide section or anarrow-band filter such as one or more ring resonator filters in anall-pass transmission phase control configuration. The activeintra-cavity transmission-mode thermo-optic optical phase tuner element130 is configured to adjust round-trip cavity phase to a constant valuewithin the compensated temperature range of laser operation, e.g. 0°Celsius (C)—85° C., based on the measured value of suitable feedbackparameter such as laser output power. The active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 providesactive control of the round trip phase of the laser operating mode, φ,and this means that the active intra-cavity transmission-modethermo-optic optical phase tuner element 130 requires power to controlthe phase. In the context of previous discussion of round trip cavityphase, the active phase tuner 130 of given length, L_(tune), powered toan elevated temperature, ΔT_(tune), over the ambient temperature T_(amb)controls the operating mode frequency to a constant value, ω_(m)′, thatis independent of T_(amb) by adjusting ΔT through the effectivethermo-optic coefficient of the tuner,

${\frac{n_{tune}}{T}\text{:}\mspace{14mu} \omega_{m}^{\prime}} = {\frac{\left( {m + 1} \right)\pi \; c}{\begin{matrix}{{{n_{{III}\text{-}V}\left( T_{amb} \right)}L_{{III}\text{-}V}} +} \\{{{n_{Si}\left( T_{amb} \right)}{L_{Si}\left( T_{amb} \right)}} + {\Delta \; T_{tune}\frac{n_{tune}}{T}L_{tune}}}\end{matrix}}.}$

The intra-cavity optical element 140 is a band-reflect grating withpassive phase compensation that is the laser output coupler whilereducing the net round trip phase change as a function of temperature.The relevant design range for the in-band reflectance is between 5% and80%. As discussed for element 120, full-width half-maximum (FWHM)reflectance bandwidth of the band-reflect grating with passive phasecompensation 140 must be less than double the free-spectral range of theband-pass optical filter 120.

The passive phase compensation properties of the band-reflect grating140 is accomplished by designing the temperature dependence of theeffective mirror position, z″(ω,T), to result in a shorter effectivesilicon cavity length, L_(Si)(T), with increasing temperature tocompensate for the positive effective thermos-optic coefficients of theIII-V and silicon waveguides,

$\frac{n_{{III}\text{-}V}}{T}\mspace{14mu} {and}{\mspace{11mu} \;}\frac{n_{Si}}{T}$

respectively. The desired design condition is then:

${{- \frac{L_{Si}}{T}}n_{Si}} \approx {{\frac{n_{Si}}{T}L_{Si}} + {\frac{n_{{III}\text{-}V}}{T}L_{{III}\text{-}V}}}$

This design methodology bounds the round trip phase of the desired laserlongitudinal operating mode to within a small total phase change rangefor a specific designed operating temperature range. Continuoussingle-mode operation requires that the remaining round trip phasechange is within the control range of the active intra-cavitytransmission-mode thermo-optical phase tuner 130, e.g. 4π, over thecompensated temperature range of laser operation, e.g. 0° C.-85° C.Assuming that the cavity round trip phase change is monotonic withtemperature, the example case can then be expressed as:

|φ(ω_(m),85° C.)−φ(ω_(m),0° C.)|<4π

The design of the passive phase compensation can be understood throughthe temperature dependence of the grating's effective mirror positionand therefore L_(Si) z″(ω_(m),T). For a uniform grating, the effectivemirror position from the input of the grating, L_(eff) (ω,T), can bewritten in terms of the coupling strength, κ(ω,T), and total gratinglength L_(g) as:

${L_{eff}\left( {\omega,T} \right)} = {\frac{1}{2{\kappa \left( {\omega,T} \right)}}{\tanh \left\lbrack {{\kappa \left( {\omega,T} \right)}L_{g}} \right\rbrack}}$

For the simplest form of thermal compensation, the coupling strengthtemperature dependence can κ(ω,T) maximized such that the effectivegrating length is reduced to compensate for the positive thermo-opticcoefficient of the rest of the laser cavity. This level of compensationmay be sufficient for short laser cavities with strongly reflectinggratings.

Stronger compensation of the net cavity thermo-optic coefficient can beachieved with properly designed chirped gratings. In a chirped grating,the effective index, n, and the grating pitch, Λ, can be varied as afunction of position. For a given frequency, ω, the grating pitch thatresults in maximum reflectance, Λ^(max)(ω,n), is approximately:

${\Lambda^{\max}\left( {\omega,n} \right)} = \frac{\pi \; c}{\omega \cdot n}$

The design of a linear chirped grating for passive phase compensation ofthe round trip cavity phase is considered in the context of the previousvariable definitions. The effective index in the grating will beapproximated as constant and equal to the unperturbed silicon cavity,n_(Si) (T). The grating pitch as a function of position will then bewritten in terms of a chirp rate,

$\frac{\Lambda}{z},$

and central pitch corresponding to the maximum reflectance condition forthe nominal operating mode, ω_(m), at reference temperature T₀:

${\Lambda (z)} = {{\frac{\Lambda}{z}\left( {z - \frac{L_{g}}{2}} \right)} + {\Lambda^{\max}\left( {\omega_{n},{n_{Si}\left( T_{0} \right)}} \right)}}$

To simplify the analysis, we can treat the effective mirror position asbeing defined as the point where the grating pitch maximizes thereflectance for the operating mode angular frequency ω_(m) attemperature T. We are then interested in obtaining the resulting changein silicon cavity length, L_(Si), with temperature that this effect canprovide. Substituting variables from the previous equations, taking thederivative with respect to temperature and neglecting higher orderterms, we can obtain the following relation:

$\frac{L_{Si}}{T} \approx {{- \left( \frac{\pi \; c}{\omega_{m}{n_{Si}^{2}\left( T_{0} \right)}} \right)}\frac{{n_{Si}}/{T}}{{\Lambda}/{z}}}$

Utilizing the previous design criteria for the passive thermo-opticphase compensation criteria to enable thermally-insensitive laseroperation, the required grating chirp parameter can then be approximatedas:

$\frac{\Lambda}{z} \approx \frac{\left( \frac{\pi \; c}{\omega_{m}{n_{Si}^{2}\left( T_{0} \right)}} \right){{n_{Si}}/{T}}}{{L_{Si}{{n_{Si}}/{T}}} + {L_{{III}\text{-}V}{{n_{{III}\text{-}V}}/{T}}}}$

This approximate chirp parameter is derived and provided to provide aconcrete design example but is not the rigorous criteria for thedisclosed laser cavities. Both the coupling and effective indextemperature dependences must be considered to choose the correct chirpparameter. Generally, the required chirp parameter results in a“red-chirped” grating such that dΛ/dz is a positive value. It should benoted that this criteria is opposite to traditional external cavitychirped grating designs that choose a negative dΛ/dz to improve noisecharacteristics. The disadvantage of a positive dΛ/dz chirp design inthis configuration is mitigated by the large longitudinal laser cavitymode free spectral ranges enabled by the compact integrated cavitydesign.

The laser beam (output) of the laser system on the silicon chip 100 ismonitored by a power monitor 18. The power monitor 18 is coupled to thewaveguide 20. Power monitoring in the laser system on silicon chip 100is utilized for control of the intra-cavity phase, to maintain efficientsingle-mode operation, for error-free link operation, and for using thelaser across the operating temperature (e.g., 0-85° C.). The powermonitor 18 can be intra-cavity (i.e., in the laser cavity 24) and/orafter the output coupler band-reflect grating with passive phasecompensation 140. In one case, having the power monitor 18 after theoutput coupler band-reflect grating with passive phase compensation 140but prior to any other integrated system components is the betterimplementation (but is not a necessity). The power monitor 18 can be anormal detector that is butt coupled to a small tap, e.g., 1%directional coupler, from the output waveguide 20, and/or an inlinepower detector such as a lateral silicon PIN diode that collectsphotogenerated carriers from defect state absorption.

FIG. 2 illustrates the single frequency cavity diagram of the laser onthe silicon chip 100 according to another embodiment. The singlefrequency cavity diagram of the laser on the silicon chip 100 in FIG. 2is identical to FIG. 1 except that the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 is omitted such that the laser outputcoupler grating is defined by a standard band-reflect grating 150 withall other reflectance characteristics shared with element 140.

As noted above in FIG. 1, the silicon chip 100 in FIG. 2 includes theintra-cavity transmission-mode optical band-pass filter 120 with afull-width half-maximum (FWHM) equal to or less than double thefree-spectral range of the cavity Fabry-Perot (F-P) resonances, theactive intra-cavity transmission-mode thermo-optic optical phase tunerelement 130, the output coupler band-reflect grating optical filter 140with an in-band reflectance in the range between 10% and 50% and afull-width half-maximum (FWHM) bandwidth equal to or greater than doublethe free-spectral range of the band-pass optical filter 120. The laserbeam (output) of the laser system on the silicon chip 100 is monitoredby a power monitor 18. Since the passive intra-cavity optical phasecompensation characteristic of the band-reflect grating with passivephase compensation 140 is omitted in FIG. 2, the silicon chip 100 inFIG. 2 has to place all of its reliance to maintain the same phase φ inthe active intra-cavity transmission-mode thermo-optic optical phasetuner element 130, which means that more power is required to maintainthe phase of the laser beam.

A transmission function is the product of the intra-cavitytransmission-mode optical band-pass filter 120 and Fabry-Perot (F-P)cavity. The transmission function is formally the amplitude and phasecharacteristic for various optical frequencies of a single output modegiven a unity amplitude and phase input mode. Alternatively, thetransmission function can be defined as the Fourier transform of thetransient impulse response of the optical system for the various inputand output modes of interest.

For single-mode operation with good side-mode suppression ratio, FWHM ofthe intra-cavity transmission-mode optical band-pass filter 120 shouldbe less than the Fabry-Perot (F-P) free spectral range (FSR). A lowerratio (between the intra-cavity transmission-mode optical band-passfilter 120 and Fabry-Perot free spectral range (FSR)) is better. Thefree spectral range (FSR) is the spacing in optical frequency orwavelength between two successive reflected or transmitted opticalintensity maxima or minima of an interferometer or diffractive opticalelement.

In traditional tunable lasers, the cavity length is adjusted whilemoving the intra-cavity filter wavelength (such as by turning adiffraction grating) to match the F-P and filter mode. Failure tosynchronously adjust the two (the cavity length and the intra-cavityfilter wavelength) results in mode-hopping or multi-mode operation.

As noted herein, temperature changes cause the wavelength/phase of thelaser beam to change. In accordance with embodiments, temperatureinsensitive laser operation is provided by cavity design and/or activecontrol. The intra-cavity transmission-mode optical band-pass filter 120wavelength is held constant throughout operation (outside of laserstartup initialization) through active control (active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130) and/orathermal design (the passive intra-cavity optical phase compensationcharacteristic of the band-reflect grating with passive phasecompensation 140). The peak transmission angular frequency of theintra-cavity filter, ω_(f), is then considered to be constant throughoutoperation and independent of ambient temperature. The lasing mode of theFabry-Perot cavity (i.e., laser cavity 24) is locked to the intra-cavitytransmission-mode optical band-pass filter 120 through active control ofthe intra-cavity phase within the compensated round-trip phase rangeacross temperature by maximizing output power as measured byintra-cavity or extra-cavity optical power monitor 18. This can beunderstood by considering the transmission ratio, TR, between theoptical band-pass filter transmissions of intra-cavity filter at theresonance angular frequency, T_(filter)(ω_(f)), and at the angularfrequency of the operating laser mode, T_(filter)(ω_(m)):

${TR} = \frac{T_{filter}\left( \omega_{f} \right)}{T_{filter}\left( \omega_{m} \right)}$

Since any value of TR greater than 1 results in a reduction in the laseroutput power relative to the case where ω_(m)=ω_(f), which is thedesired operating condition for a stabilized laser operating frequencythat is temperature independent. Ensuring a less than 1:1 ratio of thefilter FWHM (in the intra-cavity transmission-mode optical band-passfilter) to Fabry-Perot mode spacing (FSR) guarantees a strong outputpower dependent error-signal for robust control of intra-cavity phase.This condition also ensures that the TR of ω_(m) is always less than theTR for ω_(m−1) or ω_(m+1) for TR<2, ensuring that the feedback loop hasa sufficiently large error signal to continuously control operation in asingle longitudinal mode of the laser cavity over the operatingtemperature range. Based on monitoring the power monitor 18, the activeintra-cavity transmission-mode thermo-optic optical phase tuner element130 adjusts the phase of the light in the laser cavity 24 and thereforecontrols ω_(m) as described by equation 53. The lasing wavelength canthen be maintained throughout the temperature range of operation withoutundergoing changes of the Fabry-Perot mode order (i.e., without modehops) to maintain error-free link operation.

Note that sub-headings are provided below for ease of understanding andnot limitation.

Passive Thermo-Optic Phase Compensation

FIG. 3A illustrates a graph 300 showing how the III-V gain region (ofthe III-V chip 10) changes the phase of the laser beam with a change inoperating temperature. As a result of an increase in temperature on thesubstrate 30, there is an increase in phase (i.e., phase change) inIII-V gain region waveform 302 and an increase in phase (i.e., phasechange in a state-of-the-art silicon passive cavity result in a waveform305. Both waveforms 302 and 305 show an increase in phase with anincrease in temperature of the silicon chip 100. However, the passivephase compensation waveform 310 of the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 is passively configured to compensate forthe phase change by the III-V gain region (of the III-V chip 10) andcompensates for the resulting typical silicon passive cavity phasechange (of waveform 305). A state-of-the-art system would require activephase control (i.e., outside power) to compensate for the increase inphase shown in FIG. 3A but FIG. 1 does not (necessarily) require activephase control although active intra-cavity transmission-modethermo-optic optical phase tuner element 130 can be (optionally)utilized. Even when active intra-cavity transmission-mode thermo-opticoptical phase tuner element 130 is utilized in the silicon chip 100 inFIG. 1, less power is required by the active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 becausethe passive intra-cavity optical phase compensation characteristic ofthe band-reflect grating with passive phase compensation 140 compensatesfor the phase change.

FIG. 3B illustrates a schematic of the passive intra-cavity opticalphase compensation characteristic of the band-reflect grating withpassive phase compensation 140 with details of the passive thermo-opticphase compensation according to embodiments. In one case, the passiveintra-cavity optical phase compensation characteristic of theband-reflect grating with passive phase compensation 140 may be adistributed reflector grating that has smaller pitch on the left side tocompensate for the increase in phase (of the light) corresponding to theincrease in temperature of the silicon chip 100. The left side of theband-reflect grating with passive phase compensation 140 is closer tothe III-V chip 10 and pointed to the III-V chip 10, while the right sideis further away from the III-V chip 10. The pitch (linearly and/orgradually) increases from left to right (smaller pitch to wider pitch),such that the wider pitch on the right side compensates for the decreasein phase corresponding to the decrease in temperature (of the siliconchip 100). Accordingly, as the temperature increases and/or decreases(within the operation temperature (e.g., 0-85° C.) on the substrate 30of the silicon chip 100, there is a corresponding pitch variation (fromsmall pitch through wide pitch) to match the change in phase/wavelengthin the passive intra-cavity optical phase compensation characteristic ofthe band-reflect grating with passive phase compensation 140. Thesmaller pitch on the left side reflects the light with the hightemperature (higher phase and smaller wavelength), while the wider pitchon the right side reflects the light with the lower temperature (lowerphase and longer wavelength).

Intra-Cavity Filters

FIG. 4 illustrates an example intra-cavity transmission-mode opticalband-pass filter 120 according to an embodiment. In one implementation,the intra-cavity transmission-mode optical band-pass filter 120 may beany configuration of a ring-resonator, such as a Mach-Zehnder and/orgrating transmission-mode filter with appropriate free-spectral range(FSR) and bandwidth suitable for laser cavity construction. In thisimplementation, FIG. 4 shows the ring-resonator with connected waveguide20 for input and output of the light. The ring-resonator has a heater(e.g., a resistor or resistive element) that receives power in order tocontrol the temperature of the ring-resonator. The filter resonancefrequency of the ring-resonator is to be maintained throughout laseroperation. In the example intra-cavity transmission-mode opticalband-pass filter 120, FIG. 4 shows a front-up approach which is afirst-order ring resonator filter that is thermally controlled (thuscontrolling the ring resonance frequency) to a constant temperatureabove the maximum designed operation ambient temperature of the lasersystem in the silicon chip 100.

A ring-resonator, also referred to as an optical ring resonator, is aset of waveguides in which at least one is a closed loop coupled to somesort of light input and output. These can be, but are not limited tobeing, waveguides. The concepts behind optical ring resonators use lightand obey the properties behind constructive interference and totalinternal reflection. When light of the resonant wavelength/frequency ispassed through the loop from input waveguide, the light builds up inintensity over multiple round-trips due to constructive interference andis output to the output bus waveguide which serves as a detectorwaveguide. Because only a select few wavelengths will be at resonancewithin the loop, the optical ring resonator functions as a filter.Additionally, two or more ring waveguides can be coupled to each otherto form an add/drop optical filter.

Active Round-Trip Phase Control

FIGS. 5A and 5B show two different examples of the active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 accordingto an embodiment. Although examples are provided, any method of phasecontrol is suitable if the total range is approximately (˜) 4π.

FIG. 5A illustrates an implementation (which may be preferred but is nota necessity) of the active intra-cavity transmission-mode thermo-opticoptical phase tuner element 130 as a broadband thermo-optic tunerbecause the zero amplitude response and ease of control of broadbandthermo-optic tuner. The broadband thermo-optic tuner has a waveguide 20in which the light travels in and out, and a heater 505. Current can beapplied to the heater 505 to actively control the round-trip phase.

FIG. 5B illustrates another implementation of the active intra-cavitytransmission-mode thermo-optic optical phase tuner element 130 asnarrowband thermo-optic tuners (ring-resonator all-pass filters). Thenarrowband thermo-optic tuners are suitable as well, but add complexityfor resonance frequency control. The narrowband thermo-optic tuners showa waveguide 20 with two ring-resonators 510, and each ring-resonator hasa heater 505 to control the bandwidth.

Note also that carrier-injection tuners can be utilized either in thesilicon cavity and/or in the III-V die, but the carrier-injection tuneradds complexity of amplitude fluctuations.

Reflectors

FIG. 6 illustrates an example output coupler band-reflect gratingoptical filter 140 according to an embodiment. In one implementation,the output coupler band-reflect grating optical filter 140 may be astandard sidewall grating (partially etched or fully etched), which is asuitable output coupler for basic laser operation with in-bandreflectance between 10% and 50%. The in-band reflectance of the sidewallgrating is determined by III-V current-gain characteristic (of the III-Vchip 10), passive cavity loss, coupling efficiency, and output power.

The bandwidth of the sidewall grating can be designed to have at least a1 decibel (dB) suppressed reflectance for non-lasing filter order peaks(e.g., the peaks may be greater than (>) 3 dB in one case) inconfigurations without a compound intra-cavity filter characteristicthat otherwise suppresses alternate filter order transmittance. Usingthe sidewall grating, compensation of the round-trip cavity phase withreduction in effective cavity length as a function of increasingtemperature is included.

Mode Converter

FIG. 7 illustrates an example of the mode converter 16 according toembodiment. Any standard laser to silicon-on-insulator (SOI) waveguidemode coupler and packaging strategy can be applied. Requirements ofefficiency and reflectance are tied together in a requirement for stablesingle-mode operation. Ideal reflectance of 1e⁻⁵ (achievable throughangled facet interface) may be relaxed to as high as 1% in highlyefficient coupling schemes that allow the III-V die 10 HR back coatingfacet 12 and the output coupler band-reflect grating optical filter 140(e.g., output coupler grating) to dominate the cavity Fabry-Perotcharacteristic.

Power Monitors

FIG. 8 illustrates an example implementation of the power monitoraccording to an embodiment. The power monitor 18 may have an N+ dopedsilicon contact connected to a P+ doped silicon contact. The waveguide20 connects laterally through the power monitor 18. Photocurrentproportional to the waveguide power (i.e., light beam) is generated inthe power monitor 18, and the photocurrent flow perpendicular to thewaveguide 20.

Power monitoring by the power monitor 18 in the laser system isimportant for control of the intra-cavity phase to maintain efficientsingle-mode operation for error-free link operation using the proposedlaser across operating temperatures.

The power monitor 18 can be intra-cavity and/or after the output couplerband-reflect grating optical filter 140 (i.e., output coupler grating).Positioning the power monitor 18 after the output coupler band-reflectgrating optical filter 140 but prior to any other integrated systemcomponents may be the better implementation (but is not a necessity).

The power monitor 18 can be a normal detector that is butt coupled to asmall tap, e.g., 1% directional coupler, from the output waveguide 20,and/or an inline power detector such as a lateral silicon PIN diode thatcollects photogenerated carriers from defect state absorption.

Now, moving away from the sub-headings, multi-wavelength operation isdiscussed in FIGS. 9 and 10. Multi-wavelength operation may employN-port demultiplexer filters, such as Mach-Zehnder interferometer (MZI)CWDM filters developed by MZI for transceiver applications.

FIG. 9 illustrates a multi-frequency diagram of a laser on the siliconchip 100 according to an embodiment. In FIG. 9, the silicon chip 100 nowincludes a coarse N-port wavelength division multiplexing (WDM)demultiplexing filter 905 positioned (directly) after the mode converter16. The input port (IN) of the coarse N-port WDM demultiplexing filter905 connects to the mode converter 16 to receive the light, and theoutput ports (OUT) of the coarse N-port WDM demultiplexing filter 905connect to their respective 1-N integrated photonic circuits 25. As canbe seen there are multiple output ports. For example, the coarse N-portWDM demultiplexing filter 905 may receive light at different wavelengthsat the input side from the mode converter 16, such that the coarseN-port WDM demultiplexing filter 905 demultiplexes (separates) the lightby wavelength and outputs light of each wavelength to an individualoutput port. The output ports are connected to the circuits 25, and the1-N circuits 25 each include the intra-cavity transmission-mode opticalband-pass filter 120, the active intra-cavity transmission-modethermo-optic optical phase tuner element 130, and the band-reflectgrating with passive phase compensation element 140. In this case,silicon chip 100 is configured to output multiple light beams with eachat a different wavelength.

Turning to FIG. 10, another multi-frequency diagram of a laser on thesilicon chip 100 according to an embodiment. FIG. 10 is similar to FIG.9 except that the intra-cavity transmission-mode optical band-passfilter 120 is omitted because the coarse N-port WDM demultiplexingfilter 905 is sufficiently narrowband to eliminate the need for aseparate optical band-pass filter 120. In FIG. 10, the coarse N-port WDMdemultiplexing filter 905 is now a narrowband N-port WDM demultiplexingfilter that meets all the previously described parameters of theintra-cavity transmission-mode optical band-pass filter 120. FIG. 10 isa multi-frequency diagram of the laser shown in FIG. 2, and the siliconchip 100 now includes the narrowband N-port wavelength divisionmultiplexing (CWDM) demultiplexing filter 905 positioned (directly)after the mode converter 16. As noted above, the input port of thenarrowband N-port WDM demultiplexing filter 905 connects to the modeconverter 16 to receive the light, and the output ports of the coarseN-port WDM demultiplexing filter 905 connect to their respective 1-Ncircuits 25. For example, the narrowband N-port WDM demultiplexingfilter 905 may receive light at different wavelengths at the input sidefrom the mode converter 16, such that the narrowband N-port WDMdemultiplexing filter 905 demultiplexes (separates) the light bywavelength and outputs light of each wavelength to an individual outputport. The output ports are connected to the circuits 25, and the 1-Ncircuits 25 each include the active intra-cavity transmission-modethermo-optic optical phase tuner element 130 and the output couplerband-reflect grating optical filter 140. In this case, silicon chip 100is configured to output multiple light beams with each at a differentwavelength. Besides having the narrowband N-port WDM demultiplexingfilter 905 included and removing the intra-cavity transmission-modeoptical band-pass filter 120, the silicon chip 100 in FIG. 10 operatesas discussed in FIG. 1.

FIG. 11 illustrates a method 1100 of configuring the semiconductor chip100 according to an embodiment. Reference can be made to FIGS. 1, 2, 9,and 10. At block 1105, the optical gain chip 10 is attached to asemiconductor substrate 30.

At block 1110, the integrated photonic circuit 25 is provided on thesemiconductor substrate 30, and the optical gain chip 10 opticallycoupled to the integrated photonic circuit 25 forms the laser cavity 24.

At block 1115, the integrated photonic circuit 25 comprises the outputcoupler band-reflect optical grating filter with passive phasecompensation 140, the active intra-cavity thermo-optic optical phasetuner element 130, and the intra-cavity optical band-pass filter 120.

At block 1120, the output coupler band-reflect optical grating filterwith passive phase compensation 140, the active intra-cavitythermo-optic optical phase tuner element 130, and the intra-cavityoptical band-pass filter 120 are optically coupled together.

The mode converter 16 is coupled between the optical gain chip 10 andthe integrated photonic circuit 25. The output coupler band-reflectoptical grating filter with passive phase compensation 140 is configuredto reduce a net round trip phase change to within 4π over a temperaturerange. The temperature range is 0-85° Celsius.

The output coupler band-reflect optical grating filter with passivephase compensation 140 comprises a distributed reflector grating element(e.g., as shown in FIG. 3). The distributed reflector grating elementhas a smaller pitch at a first end and a wider pitch at the second end.The distributed reflector grating element is configured to shorten aneffective cavity (length) of the laser cavity 24 with increasingtemperature through increased index contract. The distributed reflectorgrating element has an elongated direction (e.g., length) and a widthdirection. The distributed reflector grating element changes in pitchalong the elongated direction (e.g., length) such that the distributedreflector grating element varies from the smaller pitch at the first endand increases to the wider pitch at the second end.

When the N-port demultiplexing filter 905 is included the silicon chip100, the silicon chip 100 includes output coupler band-reflect opticalgrating filter with passive phase compensation 140 (as shown in FIGS. 9and 10). Although FIGS. 9 and 10 show different implementations. TheN-port demultiplexing filter 905 is configured to provide differentwavelengths of light to individual ones of the plurality of (1-N)integrated photonic circuits 25. In FIGS. 9 and 10, the mode converter16 is coupled between the optical gain chip 10 and the coarse N-port WDMdemultiplexing filter 905.

FIG. 12 illustrates integrated tunable lasers for coarse wavelengthdivision multiplexing (CWDM) multiplexer transceivers on silicon, e.g.,on the silicon chip 100, according to an embodiment. The silicon chip100 is a laser or laser system as discussed above. The silicon chip 100is a laser transmitter (transmitter module), which can connect (i.e.,couple) to a laser receiver (receiver module) discussed further herein.

The silicon chip 100 has the III-V chip 10 mounted on the substrate 30,and the III-V chip 10 has the high rear reflective (HR) coating facet 12on one end and has antireflective (AR) coating facet 14 on the otherend. The mode converter 16 couples the III-V chip 10 to the waveguide20. The waveguide 20 connects to various intra-cavity optical elementsin the external integrated photonic circuit 25, which may include theintra-cavity transmission-mode optical band-pass filter 120, the activeintra-cavity transmission-mode thermo-optic optical phase tuner element130, and/or the output coupler band-reflect grating with passive phasecompensation 140. The power monitor 18, also referenced as PM1, isconnected to the output of the laser cavity 24. The tap for the powermonitor (1) 18 may be connected to (and monitors) the outputband-reflect grating with passive phase compensation 140.

Additionally, FIG. 12 shows the external integrated photonic circuit 25coupled to a 2×2 port modulator 1202 via waveguide 20. The 2×2 portmodulator 1202 has two input ports (on the left side) and two outputports (on the right side). It is noted that the elements in the lasercavity 24 are also referred to as an external cavity laser (ECL), suchas a silicon ECL (Si-ECL). A single ECL (i.e., laser cavity 24) is shownin FIG. 12 for simplicity but discussion applies to multiple ECLs 1-M onthe same silicon chip 100 as discussed further in FIG. 19. The top inputport of the 2×2 port modulator 1202 is connected to the ECL (lasercavity 24) via the output coupler band-reflect grating with passivephase compensation 140 and the bottom input port is connected to anotherECL (laser cavity 24) not shown but discussed further herein.Particularly, the bottom input port is an add port that connects to thetop output port of another 2×2 port modulator 1202 (as shown in FIG. 19)connected to another ECL as discussed further herein. High speed data isinput to the 2×2 port modulator 1202 as either a voltage or currentsignal through the modulator driver 1203 that may be either integratedon chip 100 or externally coupled.

The top output port of the 2×2 port modulator 1202 is connected to oneinput port of the N×1 port coarse WDM multiplexer 1204. The bottomoutput port of the 2×2 port modulator 1202 is connected to a powermonitor 18 (which may be referenced as PM2 to distinguish from otherpower monitors 18 in the chain). The 2×2 port modulator 1202 helps toprevent attenuation. The 2×2 port modulator 1202 is a device configuredto manipulate properties of light beams, such as the optical power orphase. The 2×2 port modulator 1202 increases the intensity of light andmay be called an intensity modulator. Once the 2×2 port modulator 1202modulates the light beams received on the top input port and the bottominput port (add port), the 2×2 port modulator 1202 outputs asuperchannel (of light) through its top output port into the top inputport of the N×1 port coarse WDM multiplexer 1204. The superchannel iscreated by the combination of light from ECLs 1-M, e.g., 4 lasercavities 24 shown in FIG. 19. The superchannel is fed to the top inputport of the N×1 port coarse WDM multiplexer 1204. Note that each inputport of the N×1 port coarse WDM multiplexer 1204 receives a superchannel(also referred to as a super-channel). Although the coarse WDMmultiplexer 1204 is shown, it is contemplated that a dense wavelengthdivision multiplexing multiplexer (DWDM) may be utilized. The N×1 portcoarse WDM multiplexer 1204 has N input ports on the left side and oneoutput port on the right side. A power monitor 18 (referenced as PM3)taps into the one output port of the N×1 port coarse WDM multiplexer1204 in order to monitor the output of the N×1 port coarse WDMmultiplexer 1204.

A superchannel is an evolution in dense wavelength division multiplexing(DWDM) (or coarse wavelength division multiplexing) in which multiple,coherent optical carriers are combined to create a unified channel of ahigher data rate, and the superchannel is brought into service in asingle operational cycle. The light output from various ECLs (lasercavities 24) is combined by a concatenation of 2×2 port modulators 1202(via the add port) in FIG. 19 in order to output a superchannel into asingle input port of the N×1 port coarse WDM multiplexer 1204. Ratherthan a single wavelength line card of greater than (>)100 gigabits persecond (Gbit/s) data rates, a superchannel creates a multi-wavelengthsignal in which each wavelength operates at the maximum data ratepermitted by commercially available analog-to-digital converter (ADC)components. The benefits of a superchannel approach are increasedspectral efficiency (a consequence of coherent detection) andoperational scalability (the ability to bring larger units of long hauloptical capacity into service for a given operational effort).

The N×1 port coarse WDM multiplexer 1204 is a device configured to use amultiplexing technique working in the wavelength domain. Wavelengthdivision multiplexing is a technique where optical signals withdifferent wavelengths are combined, transmitted together, and separatedagain. The N×1 port coarse WDM multiplexer 1204 is configured to set aparticular wavelength. The N×1 port coarse WDM multiplexer 1204 isconfigured to receive a single superchannel (of laser light) at each ofits N input ports, combine (multiplex) the superchannels (of laserlight), and output the combined superchannels (of light) over the passband of the N×1 port coarse WDM multiplexer 1204.

A thermistor 1222 is configured to measure the temperature of thesubstrate 30 on the silicon chip 100. A current output digital-to-analogconverter (DAC) 1224 is utilized to power (e.g., provide electricalcurrent) the intra-cavity transmission-mode optical band-pass filter120, and the DAC 1224 is controlled by a microcontroller 1220. Thesilicon chip 100 may include a memory array 1225, e.g., such as ane-fuse array. The microcontroller 1220 is connected to (not shown forthe sake of conciseness), controls (directly and/or indirectly), andreceives feedback from the elements (120, 130, 140, 18 (PM1, PM2, PM3),1202, 1204, 1222, 1224, 1225, 1226) shown in FIG. 12.

The microcontroller 1220 is configured to perform a self-calibrationroutine and perform normal operation control as discussed furtherherein. The microcontroller 1220 includes one or more processingcircuits (e.g., processors) configured to process/execute instructionsand control the silicon chip 100 as discussed herein. Themicrocontroller 1220 is shown on the silicon chip 100. In another case,the microcontroller 1220 may be off chip (i.e., not on the silicon chip100) but connected to the silicon chip 100.

The silicon chip 100 in FIG. 12 is configured for tuning the ECLs (e.g.,the laser cavity 24) to transmit light (wavelength) within the pass-bandof the N×1 CWDM multiplexer 1204 while accounting for temperatureinsensitivity and superchannel construction according to an embodiment.The laser light generated and controlled within the pass-band of the N×1CWDM multiplexer 1204 is at target wavelengths.

The lasing wavelength is set by the intra-cavity transmission-modeoptical band-pass filter 120 (e.g., a high-Q bandpass optical filter).The cavity configuration of the laser cavity 24 (ECL) may utilize athermally tuned ring resonator filter (as the intra-cavitytransmission-mode optical band-pass filter 120) to set the lasingwavelength. Filter resonance wavelength (i.e., lasing light wavelength)is a function of temperature and deviates from designed frequency due tofabrication process variation. Embodiments are configured to tune to theallowable target wavelengths in the N×1 port CWDM multiplexer 1204 whileself-calibrating out the fabrication process variations in the N×1 portcoarse WDM multiplexer 1204. In other words, the laser output of thelaser cavity 24 is tuned to be compatible with the N×1 port CWDMmultiplexer 1204. Although a CWDM is shown, it is understood that theN×1 port CWDM multiplexer 1204 may be a dense wavelength divisionmultiplexing (DWDM) multiplexer as understood by one skilled in the art.

FIG. 13 illustrates laser wavelength tuning within integrated WDMtransmitters. In FIG. 13, graph 300 shows the pass band of aconventional CWDM and graph 305 shows the pass band of a superchannelDWDM/CWDM.

The graphs 300 and 305 show transmission through the CWDM/DWDM on they-axis and show wavelength on the x-axis. Each graph 300 and 305 has apass band waveform 310 shown for the CWDM and/or DWDM, and the pass bandshows the group wavelengths with the highest transmission.

In graph 300, a light beam of a single laser may have a wavelength thatcorresponds to the peak of the pass band waveform 310, which means thatthat the light passes through the CWDM and is not blocked. In graph 300,the wavelength of the single laser has large window in which thewavelength may move left (decrease) or right (increase) along the x-axisand still remain in the pass band of the CWDM.

However, a superchannel does not have a single wavelength of light buthas many wavelengths of light each corresponding to different lasers(i.e., multiple ECLs on silicon chip 100). In graph 305, the differentwavelengths each have to pass through the pass band of the pass bandwaveform 310 without being blocked. In graph 305, the group ofwavelengths of the different lasers (ECLs 1-M) has very little room toshift left or right in wavelength along the x-axis before somewavelengths are blocked. In other words, the lasing wavelength must becontrolled/tuned during operation to be within the pass band of the CWDMmultiplexer transmission function for each channel by thermal tuning ofthe intra-cavity filter resonance according to embodiments. The broadpass band of the CWDM grid allows for multiple DWDM bands assumingproper control of laser wavelengths to enable N×M superchanneltransmitters according to embodiments. As will be seen in FIGS. 17A and17B, each ECLs 1-M generates and has to tune to its own targetwavelength λ_(target) 1-M within the pass band of the N×1 CWDMmultiplexer 1204.

Since inputting superchannels (containing multiple wavelengths) to theN×1 port CWDM multiplexer 1204 requires that all of the wavelengths oflight be within the pass band of the N×1 port CWDM multiplexer 1204(i.e., at the peak of transmission in the waveform 310), the siliconchip 100 in FIG. 12 is configured to tune the wavelength of each lasercavity 24, which is an ECL, to be within the pass band of the N×1 portCWDM multiplexer 1204 while taking into account temperature changes (asdiscussed above) and fabrication variations in the N×1 port CWDMmultiplexer 1204 (as discussed below).

Accordingly, FIGS. 12 (and FIG. 19) is configured to perform and/orincorporate the following, via microcontroller 1220:

(1) Utilize manufacturing consistency of thermal impedance and ambienttemperature uniformity to enable open-loop control of the laserwavelength based on a 1-time programmed on-chip e-fuse array (such asthe memory array 1225) and based on self-testable parameters in averification test (i.e., self-calibration routine/test discussed inFIGS. 14A and 14B). The verification test may be performed onwafer-scale.

(2) On chip test parameter is the transmission of the laser (ECL/lasercavity 24) through the CWDM channel pass band for the transmitter.

(3) 3 on-chip power monitors 18 (PM1, PM2, PM3) are required for theself-calibration routine (self-test) and are also useful for transceiveroperation monitoring for link health.

(4) DAC drive codes (of the current output digital-to-analog converter(DAC) 1224) for the intra-cavity filters are calculated from initializedparameters and the ambient temperature as measured by the on-chipmonitor thermistor 1222 or bandgap reference. The microcontroller 1220controls power to the intra-cavity transmission-mode optical band-passfilter 120, active intra-cavity transmission-mode thermo-optic opticalphase tuner element 130, and/or the output coupler band-reflect gratingwith passive phase compensation 140 via one or more DACs 1224.

(5) CWDM passband temperature dependence and fabrication uncertainty aredetermined in order to determine minimum guard bands (such as, e.g.,guard bands 1655, 1660 in FIGS. 16A, 16B and in FIGS. 17A, 17B) fortransceiver/receiver operation. Note that the silicon chip 100 in FIGS.12 (and FIG. 19) is a transmitter and/or transceiver.

Table 1 provides definitions that can be utilized to discuss the initiale-fuse self-calibration routine in FIGS. 14A and 14B (also referred toas a self-test, verification, calibration, etc.) and the normaloperation control (in FIG. 15) of the laser on the silicon chip 100.Table 1 is provided below.

TABLE 1 Definitions T_(amb)-Local ambient temperature as measured byon-chip thermistor/other integrated temperature sensor. (Kelvin (K)units) T_(transmaxop)-Transceiver maximum designed operating temperaturereferenced to the position of the T_(amb) sensor (K units) T_(cal)-Theambient temperature measured during the self-calibration routine (Kunits) ΔT_(calbuffer)-Excess required during calibration such that theoperating filter temperature will always be above ambient (K units) λ⁰_(filter)-Wavelength of the initial in-band intra-cavity filter order(nanometer (nm) units) λ⁻¹ _(filter)-Wavelength of the next higher orderfilter mode (nm units) λ_(target)-Target laser operation wavelength (nmunits) σ-Standard deviation of CWDM wavelength variation that resultsfrom fabrication uncertainty (nm units). λ_(red)-Wavelength of the redCWDM passband edge at the calibration temperature + 3σ fabricationuncertainty (nm units) λ_(blue)-Wavelength of the blue CWDM passbandedge at the calibration temperature-3σ fabrication uncertainty (nmunits) κ-Known thermal impedance of the intra-cavity filter relative tothe local ambient temperature T_(amb) in (K/watts (W) units-may beimplemented in terms of unitless DAC codes) δ-Intra-cavity filter heatercontrol DAC code converted to power (W units-may be implemented in termsof unitless DAC codes) δ_(max)-Maximum power DAC is capable ofoutputting for the intra-cavity filter (W units-may be implemented interms of unitless DAC codes) δ_(mincal)-Minimum filter calibration DACpower = (T_(transmaxop) + ΔT_(calbuffer) − T_(amb))/κ (W units-may beimplemented in terms of unitless DAC codes) δ_(cal)_red-DAC power for3-dB CWDM passband transmission calibration point on the red edge (Wunits-may be implemented in terms of unitless DAC codes)δ_(cal)_blue-DAC power for 3-dB CWDM passband transmission calibrationpoint on the blue edge (W units-may be implemented in terms of unitlessDAC codes) dλ/dδ-Change in filter wavelength as a function of the changein DAC power (nm/W units) δ_(offset)-DAC power offset from the nominalcalibration value to allow offsets of multiple lasers to a singlecalibration point or channel centering to minimize loss (W units-may beimplemented in terms of unitless DAC codes) RB-Single bit to definewhether the control loop should operate relative to the red (utilizingδ_(cal)_red) or blue (utilizing δ_(cal)_blue) CWDM edge (unitless)PM1-Power measured by power monitor 1 tap of laser output waveguide (Wunits) PM2-Power measured by power monitor 2 at unused modulator portoutput (W units) PM3-Power measured by power monitor 3 tap ofmultiplexed CWDM output waveguide (W units) TR1-Tap power ratio forpower monitor 1 (unitless). The tap power ratio (also referred to as thesplit power ratio) is the amount of light that is directed from theoptical network to the power monitor 1. TR1 is predefined in advance.TR3-Tap power ratio for power monitor 3 (unitless). The tap power ratio(also referred to as the split power ratio) is the amount of light thatis directed from the optical network to the power monitor 3. TR3 ispredefined in advance. T21-Transmission through the modulator (1202) asmeasured by ((PM1*TR1- PM2)/(PM1*TR1)) T31-Transmission through the CWDMchannel passband (1204) as measured by (PM3*TR3)/(PM1*TR1)

FIGS. 14A and 14B illustrate a block diagram of the self-calibrationroutine 1400 according to an embodiment. The microcontroller 1220 isconfigured to execute and/or initiate the self-calibration routine 1400on the silicon chip 100, and receives feedback from the elements in FIG.12. The microcontroller 1220 stores all calculated and measured valuesin the memory 1225. The self-calibration routine 1400 also applies toeach individual laser cavity 24 in FIG. 19.

At block 1405, the microcontroller 1220 is configured to maximizemodulator transmission (driven with DC ‘1’ signal or any predefined,constant drive signal by the modulator driver 1203) through modulatorbias adjust (of the 2×2 port modulator 1202), by using transmissionfeedback (T21) through the 2×2 port modulator 1202 as measured by thepower monitor (PM2) 18. The microcontroller 1220 is configured to applya modulator bias (e.g., voltage) via a modulator bias controller (MBC)in order to maintain a maximum transmission throughput in the 2×2 portmodulator 1202. The maximum transmission throughput in the 2×2 portmodulator 1202 allows the maximum amount of laser light to pass through.In the case when the 2×2 port modulator 1202 is narrowband, themicrocontroller 1220 is configured to maintain T21 using/receivingmodulator bias feedback through power monitor (PM2) 18 (as understood byone skilled in the art) throughout the subsequent calibration steps.

At block 1410, the microcontroller 1220 is configured to stepintra-cavity filter (current output) DAC 1224 up to the minimum filterDAC power (δ_(mincal)) while monitoring transmission (T31) through theCWDM channel pass band (of the of the N×1 port CWDM multiplexer 1204) asmeasured by the power monitor 3 (PM3) 18, all while the microcontroller1220 records the maximum transmission T31 value and current transmissionT31 value in memory 1225. For example, the microcontroller 1220 causesthe current output DAC 1224 to continuously increase electrical current(via DAC 1224) to the heater of the intra-cavity transmission-modeoptical band-pass filter 120 until the current output of the DAC 1224reaches δ_(mincal) (which may be the initial minimum calibration powerset in advance). Increasing the electrical current to the intra-cavitytransmission-mode optical band-pass filter 120 causes the wavelength(i.e., mode) to correspondingly increase in the laser cavity 24. As thewavelength changes (e.g., along the x-axis in FIGS. 16 and 17) for thelaser light, the microcontroller 1220 stores the transmission throughputT31 (e.g., the transmission response on the y-axis) of the N×1 port CWDMmultiplexer 1204 at the different wavelengths.

At block 1415, the microcontroller 1220 is configured to determine/checkif the current transmission T31 (i.e., presently measured and calculatedT31) has fallen by more than 3 db relative to the maximum transmissionT31 recorded. Assuming the tap power ratio TR1 and the tap power ratioTR3 both remain constant, the value of the transmission through the CWDMmultiplexer 1204 channel pass band is based on (or changes according to)the measurements of PM3/PM1. The maximum transmission T31 has beenstored in memory 1225, along with each value of transmission T31 passingthrough N×1 port CWDM multiplexer 1204.

At block 1420, when the current transmission T31 has fallen down by morethan 3 dB compared to the maximum recorded transmission T31, themicrocontroller 1220 is configured to step up the intra-cavity currentDAC 1224 (i.e., increase the current output from the DAC 1224 to theintra-cavity transmission-mode optical band-pass filter 120) whilemonitoring T31 until T31 restores to (at least and/or within) 3 dB ofthe maximum recorded T31 value. Stepping up the electrical current tothe intra-cavity transmission-mode optical band-pass filter 120 servesto switch the operating laser mode (wavelength) from λ⁰ _(filter) to λ⁻¹_(filter) and shift the laser operating condition from the red side tothe blue side of the CWDM pass band of N×1 port CWDM multiplexer 1204.That is, the wavelength (i.e., laser operating mode) is stepped up(increase power from DAC 1224) in order to calibrate to blue side inFIGS. 13, 16, 17.

At block 1421, the current DAC 1224 code (the present δ determined inblock 1420) is stored in local memory 1225 as the calibration powerδ_(cal) _(_) _(blue).

At block 1422, the microcontroller 1220 is configured to step up theintra-cavity current DAC 1224 (i.e., increase the current output fromthe DAC 1224 to the intra-cavity transmission-mode optical band-passfilter 120) while monitoring T31 until T31 falls to (at least) 3 dB ofthe maximum recorded T31 value.

At block 1423, the current DAC 1224 code (the present δ determined inblock 1422) is stored in local memory 1225 as the calibration powerδ_(cal) _(_) _(red).

In the alternative case of 1415, at block 1425, when the currenttransmission T31 (presently monitored/calculated) has not fallen by morethan 3 dB compared to the maximum recorded transmission T31 over thesweep up to δ_(min), the microcontroller 1220 is configured to step theintra-cavity filter current DAC 1224 (i.e., increase the current outputfrom the DAC 1224 to the intra-cavity transmission-mode opticalband-pass filter 120) while monitoring T31 until transmission T31 fallsto 3 dB of the maximum recorded transmission T31 value. Now, the laseris operating on the red side of the CWDM passband.

At block 1426, the current DAC 1224 code (the present δ determined inblock 1425) is stored in local memory 1225 as the calibration powerδ_(cal) _(_) _(red).

At block 1427, the microcontroller 1220 is configured to step up theintra-cavity current DAC 1224 (i.e., increase the current output fromthe DAC 1224 to the intra-cavity transmission-mode optical band-passfilter 120) while monitoring T31 until T31 restores to (within) 3 dB ofthe maximum recorded T31 value.

At block 1428, the current DAC 1224 code (the present δ determined inblock 1427) is stored in local memory 1225 as the calibration powerδ_(cal) _(_) _(blue).

At block 1430, the microcontroller 1220 is configured to store in thememory 1225 (e.g., e-fuse array) the following parameters:

1) store T_(amb) as the calibration temperature T_(cal);

2) store the local memory value δ_(cal) _(_) _(blue) as calibrationpower δ_(cal) _(_) _(blue); and

3) store the local memory value δ_(cal) _(_) _(red) as calibration powerδ_(cal) _(_) _(red).

FIG. 15 illustrates a block diagram 1500 of the normal operation controlof the silicon chip 100 after the self-calibration routine 1400according to an embodiment. The microcontroller 1220 is configured toexecute the normal operation control on the silicon chip 100. Again, themicrocontroller 1220 stores all calculated and measured values in thememory 1225.

At block 1505, the microcontroller 1220 is configured to determine theDAC 1224 output power (e.g., required electrical current) fromcalibration parameters and known system parameters, where theself-calibration routine in FIGS. 14A and 14B enable feedback-free laserwavelength control to speed system initialization and eliminatedither/loop instability. That is, once the self-calibration routine 1400has been executed for each ECL (laser) and feedback is received by themicrocontroller 1220, the self-calibrated ECLs 1-M are able to run withfeedback-free laser wavelength control. The DAC 1224 output power is theelectrical current that needs to be applied to the intra-cavitytransmission-mode optical band-pass filter 120 to maintain the targetwavelength within the pass band of the N×1 port CWDM multiplexer 1204while taking into account fabrication variation of the to the N×1 portCWDM multiplexer 1204 (as determined during the self-calibration routine1400).

At block 1510, the microcontroller 1220 is configured to read the bit(RB) in the memory 1225 to determine whether the bit (RB) has been setto the red edge or blue edge in FIGS. 14A and 14B.

At block 1515, when the bit (RB) is set to the red edge, themicrocontroller 1220 is configured to perform red edge calibration(where the red edge is defined for the highest end of uncertainty):δ=δ_(cal) _(_)_(red)−δ_(offset)−((λ_(red)−λ_(target))/dλ/dδ)−((T_(amb)−T_(cal))/κ).Accordingly, the target wavelengths generated by the ECLs are to be setat and/or below the red edge defined for the highest end of uncertainty(in FIGS. 16 and 17).

At block 1520, when the bit (RB) is set to the blue edge, themicrocontroller 1220 is configured to perform blue edge calibration(where the edge is defined for the lowest end of uncertainty): δ=δ_(cal)_(_)_(blue)+δ_(offset)+((λ_(target)−λ_(blue))/dλ/dδ)−((T_(amb)−T_(cal))/κ).Accordingly, the target wavelengths generated by the ECLs are to be setbelow and/or above the red edge defined for the lowest end ofuncertainty (in FIGS. 16 and 17).

In the above calculation of DAC 1224, output power is offset by thepreset value of δ_(offset) by the microcontroller 1220. This may bedefined by the system designer to implement options such as targetingthe operation wavelength to be close to center of the CWDM band foroptimal transmission and/or as close as possible to the blue edge tominimize tuning power. This can be coupled with the RB bit toautomatically minimize tuning power by using logic on themicrocontroller 1220 to pick the minimum required DAC 1224 output power.Alternatively, in the super-channel case in which multiple ECL channelsare tuned within a single CWDM band, optimal tuning precision can beachieved by uniformly setting the RB bit for all channels and utilizingdifferent δ_(offset) values to space the operating wavelengthsappropriately through the CWDM band.

The cavity round-trip phase is actively controlled through powerfeedback by maximizing PM1 (power monitor (1) 18) throughout operation.When the cavity round-trip phase changes, the power/current applied tothe active intra-cavity transmission-mode thermo-optic optical phasetuner element 130 increases or decreases in order to maintain theround-trip phase. Maintaining the round-trip phase maintains the laserwavelength in the laser cavity 24 (ECL). Any deviation caused byimproper round-trip phase results in reduced output power by theFabry-Perot round trip longitudinal mode frequencies deviating from thepeak of the intracavity filter transmission function. Thereforeadjusting the phase tuner element 130 maximizes output power whilemaintaining operation wavelength. Any feedback algorithm suitable formaximization applications can be implemented on the microcontroller asis known by those skilled in the art. It is noted that the activeintra-cavity transmission-mode thermo-optic optical phase tuner element130 is connected to the power monitor 18 (PM1) (via band-reflect gratingwith passive phase compensation 140) in order to provide feedback to themicrocontroller 1220 (thus maintaining the round-trip phase).

Turning to FIG. 16A, graph 1605 illustrates the allowable targetwavelength range 1650 at which to make/insert the target wavelength(λ_(target), of the ECL in order to pass through the pass band of theN×1 port CWDM multiplexer 1204, while taking into fabrication variationsin the N×1 port CWDM multiplexer 1204. FIG. 16B illustrates graph 1610that shows the allowable target wavelength range at which to make/insertthe target wavelength (λ_(target)) in order to pass through the passband of an N×1 port CWDM demultiplexer in a receiver (e.g., the N×1 portCWDM demultiplexers 2104 in a superchannel DWDM/CWDM receiver 2100 inFIG. 21), while taking into fabrication variations in the N×1 port CWDMdemultiplexer 2104. Note that the N×1 port CWDM multiplexer 1204 in thesilicon chip 100 (which is the transmitter module) has the samefabrication variations as the N×1 port CWDM demultiplexer 2104 in asilicon chip 2100 (which is the receiving module). The target wavelengthcan be defined to the microcontroller 1220 by the system designerthrough the δ_(offset) parameter.

Based on fabrication uncertainty (in the N×1 port CWDM multiplexer 1204and the N×1 port CWDM demultiplexer 2104) and temperature changes, thepass band shifts. The minimum operating temperature is from the left (onthe x-axis for wavelength) and the maximum operating temperature is tothe right (on the x-axis for wavelength). According tomanufacturing/fabrication uncertainty (i.e., manufacturing tolerances),this examples illustrates that there are six possible pass bandwaveforms 1611, 1612, 1613, 1614, 1615, 1616 (shifting from left toright in wavelength) shown in both FIGS. 16A and 16B. The targetwavelength (λ_(target)) is to fall within the allowable targetwavelength range 1650, which is within the pass band of all thewaveforms 1611, 1612, 1613, 1614, 1615, 1616. This means that the laserin the laser cavity 24 (ECL) is controlled to have a target wavelengthwithin the target wavelength range 1650, and the manufacturingtolerances (i.e., fabrication uncertainty) is known in advance. Thetarget wavelength in FIGS. 16A and 16B ensures that the laser lightgenerated by the laser cavity 24 (ECL) on the silicon chip 100 is ableto pass through the both N×1 port CWDM multiplexer 1204 and the N×1 portCWDM demultiplexer 2104 even though pass band waveforms may shift fromthe waveform 1611 at the far left to the waveform 1616 at the far right.The self-calibration routine 1400 executed by the microcontroller 1220ensures that the calibrated laser cavity 24 (ECL) stays within thetarget wavelength range 1650.

The edge transmission pass bands 1655 and 1660 may each be set to be a−1 db pass band range. This means that the edge transmission pass band1655 is a 1 db pass band range below the blue edge of the targetwavelength range 1650, while the edge transmission pass band 1660 is a 1db pass band range above the red edge of the target wavelength range1650. In another case, the edge transmission pass bands 1655 and 1660may each be set to −3 db pass band range of the target wavelength range1650. Although the target wavelength λ_(target) can operate in the edgetransmission pass bands 1655 and 1660, there may be high insertion lossin these areas. The target wavelength range/band 1650 sets the allowablewavelengths in the worst case corners with self-calibration uncertainty.The blue edge/side of the target wavelength range/band 1650 has thelowest energy consumption (by the active intra-cavity transmission-modethermo-optic optical phase tuner element 130 to control phase/wavelengthof the laser light) while the red edge/side of the target wavelengthrange/band 1650 requires the highest energy consumption.

The self-calibration routine 1400 executed by the microcontroller 1220of the silicon chip 100 determines the target wavelength range/band 1650in which the target wavelength is to be inserted. The self-calibrationroutine 1400 and the normal operation control 1500 include the thermalimpedance κ and thermo-optic coefficients dλ/dδ of the intra-cavitytransmission-mode optical band-pass filter 120 as the (required)parameters for the control loop. If these parameters (the thermalimpedance κ and thermo-optic coefficients dλ/dδ) are not stable enoughfor the desired wavelength accuracy, the parameters can be measured on alot or wafer level and then programmed into the die through the e-fusearray during the self-calibration routine as well. If narrowbandmodulators are utilized for 2×2 port modulator 1202, the free-spectralrange of the resonant 2×2 port modulator 1202 and the intra-cavitytransmission-mode optical band-pass filter 120 are to be matched.

The free spectral range of the intra-cavity transmission-mode opticalband-pass filter 120 is to be larger than the CWDM pass band full-widthat half maximum (FWHM) of the N×1 port CWDM multiplexer 1204.

The laser operating range, e.g., reflection bandwidth of the outputcoupler band-reflect grating with passive phase compensation 140, shouldbe larger than the CWDM pass band FWHM (of the N×1 port CWDM multiplexer1204) while accounting for the fabrication uncertainties of bothcomponents.

Now turning to FIGS. 18A and 18B, a method 1800 is provided forconfiguring a semiconductor chip 100 having the ECL (laser) that isself-calibrated via the self-calibration routine 1400 according to anembodiment.

At block 1805, the optical gain chip 10 attached to the semiconductorsubstrate 30 is provided. At block 1810, the integrated photonic circuit25 on the semiconductor substrate 30 is provided, such that the opticalgain chip 10 is optically coupled to the integrated photonic circuitthereby forming a laser cavity 24.

At block 1815, the modulator 1202 is coupled to the integrated photoniccircuit 25. At block 1820, the wavelength division multiplexing (WDM)multiplexer 1204 is coupled to an output of the modulator 1202, and theWDM multiplexer 1204 is calibrated to in accordance with aself-calibration routine.

The self-calibration routine is configured to be executed by themicrocontroller 1220. At block 1825, the microcontroller 1220 isconfigured to drive the modulator 1202 (by applying a modulator biasvoltage) to maximize transmission through the modulator 1202. At block1830, the microcontroller 1220 is configured to step up electrical power(via of DAC 1224) to the intra-cavity optical band-pass filter 120 up toa predefined level while continuously monitoring light transmission (viapower monitor 18 (PM3)) through the WDM multiplexer 1204 in order tostore a maximum recorded transmission (measured earlier) and a currenttransmission (presently measured and occurring) through the WDMmultiplexer 1204, where the WDM multiplexer has a WDM multiplexer passband. At block 1831, the microcontroller 1220 is configured to check thecurrent transmission (presently measured) through the WDM multiplexer1204 relative to the maximum recorded transmission and determine whetheror not the transmission has fallen by more than a predefined amount(e.g., 3 dB).

At block 1835, the microcontroller 1220 is configured to, when theoutcome of block 1831 is positive, step up the electrical power (via ofDAC 1224) to the intra-cavity optical band-pass filter 120 until thecurrent transmission (being presently measured) is restored (rises) tothe predefined amount of the maximum recorded transmission (e.g., 3 dbof the maximum recorded transmission), thereby determiningself-calibration parameters for a blue edge of the WDM multiplexer passband. The blue edge is shown in FIGS. 16A, 16B, 17A, 17B.

At block 1840, the microcontroller 1220 is configured to, when theoutcome of block 1831 is negative, step up the electrical power (via DAC1224) to the intra-cavity optical band-pass filter 120 until the currenttransmission falls (reduces) to the predefined amount of the maximumrecorded transmission (e.g., falls to the 3 db of the maximum recordedtransmission), thereby the self-calibration parameters for a red edge ofthe WDM multiplexer pass band. The red edge is shown in FIGS. 16A, 16B,17A, 17B.

At block 1845, the microcontroller 1220 stores the self-calibrationparameters in memory 1225 as determined and/or measured from theself-calibration routine 1400.

The self-calibration parameters include a bit (e.g., RB bit stored inmemory 1225) defining whether the self-calibration routine is calibratedto the blue edge and/or the red edge of the WDM multiplexer pass band ofthe WDM multiplexer 1204.

Further, the self-calibration parameters (stored in memory 1225)include: setting an ambient temperature as a calibration temperature,and setting the electrical power measured, at the predefined amount(e.g., 3 db) of the maximum recorded transmission corresponding to theblue edge or red edge of the WDM multiplexer pass band, as a calibratedelectrical power. The self-calibration parameters (stored in memory1225) are utilized to calibrate from the blue edge and/or the red edgeof the WDM multiplexer pass band.

When calibrating from red edge of the WDM multiplexer pass band (inFIGS. 16A, 16B, 17A, 17B), the microcontroller 1220 is configured todecrease the calibrated electrical power corresponding to the red edgeof the WDM multiplexer pass band in order to determine a normaloperation electrical power for operating at a target wavelengthλ_(target). For example, the DAC 1224 can decrease electrical power tothe intra-cavity optical band-pass filter 120 to drop to a targetwavelength below the red edge of the WDM multiplexer pass band (butabove the blue edge), as directed by the microcontroller 1220.

When calibrating from blue edge of the WDM multiplexer pass band (inFIGS. 16A, 16B, 17A, 17B), the microcontroller 1220 is configured toincrease the calibrated electrical power corresponding to the blue edgeof the WDM multiplexer pass band in order to determine a normaloperation electrical power for operating at a target wavelength. Forexample, the DAC 1224 can increase electrical power to the intra-cavityoptical band-pass filter 120 to increase to a target wavelength abovethe blue edge of the WDM multiplexer pass band (but below the red edge),as directed by the microcontroller 1220.

The blue edge of the WDM multiplexer pass band corresponds to a loweredge (lower wavelength) of the WDM multiplexer pass band through whichlight transmission can occur. The red edge of the WDM multiplexer passband corresponds to a higher edge (higher wavelength) of the WDMmultiplexer pass band through which light transmission can occur.

The integrated photonic circuit 25 comprises the active intra-cavitythermo-optic optical phase tuner element 130, the intra-cavity opticalband-pass filter 120, and the output coupler band-reflect opticalgrating filter with passive phase compensation 140 which are alloptically coupled together.

FIGS. 17A and 17B correspond to the description of FIGS. 16A and 16Bexcept that FIGS. 17A and 17B show multiple target wavelengthsλ_(target) 1-M, which is one target wavelength for each ECL 1-M (eachlaser cavity 24) in FIG. 19. In FIG. 17A, the graph 1605 illustrates theallowable target wavelength range 1650 at which to make/insert themultiple target wavelengths λ_(target) 1-N in order to pass through thepass band of the N×1 port CWDM multiplexer 1204, while taking intofabrication variations in the N×1 port CWDM multiplexer 1204. FIG. 17Billustrates the graph 1610 that shows the allowable target wavelengthrange 1650 at which to make/insert the target wavelength (λ_(target)) inorder to pass through the pass band of the N×1 port CWDM demultiplexer2104 (e.g., receiver module) in FIG. 21, while taking into fabricationvariations in the N×1 port CWDM demultiplexer 2104. The process offurther creating multiple target wavelengths λ_(target) 1-N is discussedbelow in FIG. 19.

Accordingly, the same principle discussed above in FIGS. 12-18 can beapplied to multiple Si-ECLS ((multiple laser cavities 24) on the samesilicon chip 100) per CWDM pass band in superchannel creation. Now,discussion turns to Si-ECL tuning for superchannel creation within CWDMpass bands. When the multiple laser cavities 24 (multiple ECLs) areutilized to create superchannels (as shown in FIG. 19), theinitialization algorithms, tuning algorithms, and requirements executedby the microcontroller 1220 (i.e., self-calibration routine and normaloperation in FIGS. 14 and 15, respectively) match the single-channelcase (typical CWDM) with some extra clarifications regarding the morecomplex application.

The benefit of the previously described self-calibration routine 1400 isthat although the CWDM pass band uncertainty may or may not be removed(completely) in the N×1 port CWDM multiplexer 1204, the fabricationuncertainty of the intra-cavity transmission-mode optical band-passfilter 120 is calibrated out (i.e., removed). The calibration procedurehas effectively determined all required control parameters to set therelative frequency alignment of the filter resonance with regard to theCWDM transmission band.

Instead of controlling for a single target wavelength (such asλ_(target) in FIGS. 16A and 16B) within the allowable range(particularly target wavelength range/band 1650) of the CWDM band in N×1port CWDM multiplexer 1204, multiple target wavelengths (such as such asλ_(target) 1-M in FIGS. 17A and 17B) can be chosen for each CWDM band atan arbitrary spacing. The wavelength spacing between each one of thetarget wavelengths λ_(target) 1-M (e.g., target wavelength 1, targetwavelength 2, target wavelength 3, target wavelength 4) can be 2 nm.This spacing is established in the control algorithm by settingdifferent δ_(offset) parameters for each λ_(target).

The multiple Si-ECL transmitters (such as ECLs 1-M) can be combinedthrough the add port of the narrowband modulators 1202 in thetransmitter schematic shown FIG. 19, using a concatenationconfiguration.

FIG. 19 illustrates a superchannel DWDM/coarse WDM laser transmitterusing Si-ECLs 1-M on a silicon chip, such as the silicon chip 100,according to an embodiment. FIG. 19 includes the features of FIG. 12,except that FIG. 19 shows multiple (laser cavities 24) ECLs 1-M on thesame silicon chip 100, where each output of each ECL 2-M isconsecutively added to the add port of the next 2×2 port modulator 1202(eventually combining with ECL 1) to generate a superchannel that isinput eventually into the N×1 port CWDM multiplexer 1204. The siliconchip 100 is a laser or laser system as discussed above. The silicon chip100 operates as a superchannel DWDM/CWDM transmitter (transmittermodule), which can connect (i.e., optically couple) to the receiver 2100in FIG. 21 (receiver module) discussed further herein.

As noted above, the silicon chip 100 has the III-V chip 10 mounted onthe substrate 30, and the III-V chip 10 has the high rear reflective(HR) coating facet 12 on one end and has antireflective (AR) coatingfacet 14 on the other end. The mode converter 16 couples the III-V chip10 to the waveguide 20. Additionally, the waveguide 20 connects theintra-cavity transmission-mode optical band-pass filter 120, the activeintra-cavity transmission-mode thermo-optic optical phase tuner element130, and/or the output coupler band-reflect grating with passive phasecompensation 140.

FIG. 19 shows the external integrated photonic circuit 25 coupled to the2×2 port modulator 1202 via waveguide 20. The 2×2 port modulator 1202has two input ports (on the left side) and two output ports (on theright side). Each one of the multiple ECLs 1-M on the same silicon chip100 has its own 2×2 port modulator 1202, where the top input port of the2×2 port modulator 1202 is connected to the respective ECL 1-M (lasercavity 24) via the output coupler band-reflect grating with passivephase compensation 140, while the bottom add input port is connected toanother previous ECL. Note that the bottom add input port of ECL M isunused. The bottom input port is the add port that connects to the topoutput port of the previous output of the 2×2 port modulator 1202 in thecascading configuration as shown in FIG. 19. This cascadingconfiguration continues through the 2×2 port modulator 1202 in ECL 1,and the 2×2 port modulator 1202 of ECL 1 outputs the combined/modulatedlight of ECL 1-M into the N×1 port CWDM multiplexer 1204.

Once the last 2×2 port modulator 1202 (of ECL 1) modulates thecontinuous wave (CW) light beam received on the top input port andpasses through the modulated light beams received at the bottom inputport (add port), the last 2×2 port modulator 1202 outputs a superchannel(of light) through its top output port into the top input port of theN×1 port coarse WDM multiplexer 1204. The superchannel is created by thecombination of light from ECLs 1-M (which are, e.g., the 4 lasercavities 24 shown in FIG. 19). The superchannel is fed to the top inputport of the N×1 port coarse WDM multiplexer 1204. Another set of ECLs1-M connects to the second input port of the N×1 port coarse WDMmultiplexer 1204 through a similar optical network. Note that each inputport of the N×1 port coarse WDM multiplexer 1204 receives a superchannel(also referred to as a super-channel). The N×1 port coarse WDMmultiplexer 1204 is configured to receive a single superchannel (oflight) at each of its N input ports, combine (multiplex) thesuperchannels (of light), and output the combined superchannels (oflight) over the pass band of the N×1 port coarse WDM multiplexer 1204.Although the coarse WDM multiplexer 1204 is shown, it is contemplatedthat a dense wavelength division multiplexing multiplexer (DWDM) may beutilized. The N×1 port coarse WDM multiplexer 1204 has N input ports onthe left side and one output port on the right side.

The power monitors 18 (PM1, PM2) monitor each of the ECLs 1-M at twoseparate locations. The power monitor 18 (PM1) is connected to theoutput of the laser cavity 24 (ECL) (i.e., connected to the outputband-reflect grating with passive phase compensation 140). The powermonitor 18 (PM2) is connected to one output port of each 2×2 portmodulator 1202. The power monitor 18 (PM3) is connected to/taps into andmonitors the superchannel output of N×1 port CWDM multiplexer 1204 thatis bound for the receiver 2100 in FIG. 21. The microcontroller 1220receives feedback from each power monitor 18 (PM1, PM2, PM3).

FIG. 19 also includes the thermistor 1222, current outputdigital-to-analog converters (DAC) 1224 for powering (e.g., provideelectrical current) each of the respective intra-cavitytransmission-mode optical band-pass filters 120, and the DACs 1224. TheDACs 1224 are controlled by the microcontroller 1220. Although not shownfor simplicity, each of the intra-cavity transmission-mode opticalband-pass filters 120 in the ECLs 1-M has its own DAC 1224. Although notshown for the sake of conciseness, the microcontroller 1220 is connectedto and controls (directly and/or indirectly) the elements shown in FIG.19.

The silicon chip 100 in FIG. 19 is configured for tuning, e.g., 4 ECLs1-M (which means 4 laser target wavelengths (λ_(targets) 1-M)) withinthe pass band of CWDM transmitter for temperature insensitivity andsuperchannel construction according to an embodiment. Therefore, thesilicon chip 100 in FIG. 19 is configured to tune the 4 targetwavelengths within the allowable target wavelength range 1650 (in FIGS.17A and 17B) in the N×1 port CWDM multiplexer 1204 whileself-calibrating out the fabrication process variations in the N×1 portcoarse WDM multiplexer 1204. In other words, the laser output of the 4laser cavities 24 (4 ECLs 1-M) is tuned to be compatible with the N×1port CWDM multiplexer 1204.

The microcontroller 1220 is configured to perform the self-calibrationroutine 1400 (in FIGS. 14A and 14B) separately for each ECL 1-M (i.e.,for each laser) such that each target wavelength λ_(target) (which isone target wavelength from each ECL 1-M) is self-calibrated to be withinthe CWDM pass band of the N×1 port CWDM multiplexer 1204, as discussedabove along with additions further herein. Then, the microcontroller1220 is configured to perform normal operation control in FIG. 15. Sinceonly one laser (one ECL in FIG. 19) can be tuned at a time during theself-calibration routine 1400, each separate intra-cavitytransmission-mode optical band-pass filter 120 must be referenced toeither the red or blue edge of the CWDM pass band of the N×1 port CWDMmultiplexer 1204 to ensure accuracy in relative frequency/wavelengthalignment. There are two implementation options that can be added to theself-calibration routine 1400. In the option, the microcontroller 1220starts the self-calibration sequence 1400 exactly as described herein.If the lasers (ECL 1-M) do not all calibrate to the same edge in FIGS.17A and 17B, the microcontroller 1220 is configured to recalibrate anylasers (ECLs 1-M) initially referenced to the blue edge to the red edgeinstead. The second option is for the microcontroller 1220 to initiallycalibrate all lasers (each ECL 1-M) to the red edge as the defaultsequence. The calibration edge can always be shifted in theself-calibration algorithm by continuing the described procedure until aconsistent edge of rising transmission or falling transmission isachieved.

The microcontroller 1220 is configured to provide channel spacingbetween each individual target wavelength generated in each ECL 1-M inFIG. 19. It is noted that each ECL 1-M generates a single targetwavelength λ_(target). As an example scenario, it may be assumed thatthere are 4 ECLs resulting in 4 target wavelengths, where M=4 (i.e., ECL1 is tuned to generate λ_(target) 1, ECL 2 is tuned to generateλ_(target) 2, ECL 3 is tuned to generate λ_(target) 3, ECL 4 is tuned togenerated λ_(target) 4). Referring back to FIGS. 17A and 17B for the 4target wavelengths λ_(target) 1-4, the microcontroller 1220 createsrelatively coarse 2 nm DWDM channel spacing within standard1270/1290/1310/1330 CWDM grids; this provides 4×4 superchanneltransceivers for 25 Gb/s×16=400 Gb/s aggregate bandwidth. The designedDWDM channel wavelength separation should account for the variability inthe control DACs 1224, along with the accuracy of the thermal impedanceand thermo-optic tuning rates. The channel separation referenced to thered or blue calibration edge in terms of wavelength is determined byδ_(offset) (dλ/dδ). The parameter of dλ/dδ has some residual uncertaintydue to variations in the thermal impedance and thermo-optic tuning ratesthat are observed in manufacturing. As a result, the wavelength spacingbetween superchannel targets should be sufficiently large that thesevariations do not result in performance degradation.

The resonant filter frequencies can either be designed to be identical(within fabrication precision) or offset with a resonant frequency steprepresentative of the DWDM grid. Offset filters may minimize totaltuning power required for stabilization but this may add complexity.

Now turning to FIG. 20, an example implementation of the 2×2 portmodulator 1202 is provided according to an embodiment. There is one 2×2port modulator 1202 per channel (i.e., per ECL 1-M). The input port ofthe 2×2 port modulator 1202 is connected to the respective ECL 1-M, andthe add port on the left input side is connected to the output from theprevious 2×2 port modulator 1202 in the cascading chain. The 2×2 portmodulator 1202 is connected to the power monitor 18 (PM2) per ECL 1-M,and the output of the 2×2 port modulator 1202 corresponding to ECL 1goes to one input port of the N×1 port CWDM multiplexer 1204.

The resonant ring modulator 2005 (e.g., a ring resonator) has a ringmodulator resonant frequency. Wavelengths away from (i.e., off resonancewith) the ring modulator resonant frequency pass from the add port tooutput without interference, and these wavelengths are off resonancelight. The light at the input port is assumed to be on resonance withthe ring modulator resonant frequency, and thus pass to the output port.For light from the add port that is on resonance with the light at theinput port, the 2×2 port modulator 1202 adds the light of the add portto the light of the input port and sends the combined light to theoutput port.

Optimizing inverse eye on power monitor 18 (PM2) enables simple feedbackoperation to the microcontroller 1220. Matching resonatorcharacteristics of intra-cavity Si-ECL filter (intra-cavitytransmission-mode optical band-pass filter 120) ease wavelengthalignment control difficulty.

FIG. 21 illustrates the superchannel DWDM/CWDM receiver 2100 (receivermodule) which receives the superchannel of light transmitted from theoutput of the N×1 port coarse WDM multiplexer 1204 in FIG. 19 accordingto an embodiment.

The superchannel CWDM receiver 2100 may be on a substrate 31. Thesubstrate 31 may have the same features as the substrate 30. Thesuperchannel CWDM receiver 2100 includes a polarization splitter rotator2105 configured to receive and split the received light of thesuperchannel (output from the transmitter silicon chip 100 in FIG. 19(or FIG. 12)). The polarization splitter rotator 2105 outputs the lightto the input of two N×1 port CWDM multiplexers 2104. The N×1 port CWDMmultiplexers 2104 demultiplexes the received light and then outputs thedemultiplexed light to four counter propagating drop filters (2120) atλ₁ through λ_(M) for dual photodiodes. The counter propagating dropfilters 2120 are channel dropping filters that access one channel (i.e.,one target wavelength λ_(target)) of a wavelength division multiplexed(WDM) signal, while not disturbing the other channels (other targetwavelength λ_(targets)). In other words, the counter propagating dropfilters single channel receive block 2120 captures the light at aparticular target wavelength and generates an electrical signal that issent to an electrical receiver 2130. Each of the four example counterpropagating drop filters 2120 has its own electrical receiver 2130. Theelectrical receivers 2130 may be on chip 100 in one case, and off chipin another case. Each counter propagating drop filter 2120 has ringresonators 2150 which connect to the respective waveguides 2165 (one percounter propagating drop filter 2120). For the particular targetwavelength λ_(target) (channel) selected by the counter propagating dropfilter 2120, the photodiodes 2155 convert the light energy into anelectrical signal that is sent to the electrical receiver 2130. For eachN outputs of the coarse WDM demultiplexer 2104, an M channel receiveblock 2125 is attached to form a complete superchannel receiver.

FIG. 22 is a method 2200 of creating one or more superchannels on asemiconductor chip 100 according to an embodiment. Numeroussuperchannels may be created using the techniques discussed herein. Eachsuperchannel is input into one of the input ports of the N×1 port coarseWDM multiplexer 1204, although the ECLs 1-M corresponding creating onesuperchannel is illustrated in FIG. 19. The N×1 port coarse WDMmultiplexer 1204 has N input ports and 1 output port.

At block 2205, a plurality of laser cavities 24 including a first lasercavity, a next laser cavity, through a last laser cavity (e.g., ECLs1-M) are formed on the semiconductor chip 100.

At block 2210, a plurality of modulators 1202 including a firstmodulator, a next modulator, through a last modulator (e.g., the 2×2port modulators 1202) are provided on the semiconductor chip 100, whereeach of the plurality of modulators 1202 has a direct input (e.g., theinput connected to a single ECL), an add port (the port connected to(and adding) the outputs from other ECLs in a daisy chain), and anoutput port (e.g., the output port of the modulator 1202 that connectsto the add port of the next modulator 1202).

At block 2215, a concatenated arrangement of the plurality of lasercavities 24 is configured to form a superchannel as shown in FIG. 19. Inthe concatenated arrangement, the last laser cavity 25 (e.g., ECL M) iscoupled to the direct input of the last modulator 1202, and the outputof the last modulator is coupled to the add port of the next modulator1202, at block 2220. The add port of the last modulator 1202(corresponding to ECL M) is not used or may not be present.

At block 2225, the next laser cavity 24 (e.g., ECL 3) is coupled to thedirect input of the next modulator 1202, and the output of the nextmodulator 1202 is coupled to the add port of the first modulator. Thesame concatenated arrangement may continue for various ECLs and theircorresponding modulators 1202 within the daisy chain until the top ofthe chain is reached. It is understood by one skilled in the art thatthe intervening arrangements a particular ECL coupling to its modulator1202 are the same, and it is assumed that the first laser cavity andfirst modulator are at the top of the daisy chain.

At the top of the daisy chain in block 2230, the first laser cavity 24(e.g., ECL 1) is coupled to the direct input of the first modulator1202, and the output of the first modulator 1202 is coupled to one inputof an N×1 port coarse wavelength division multiplexing multiplexer 1204,thus forming the superchannel (of laser light) that is input into oneinput port of the WDM multiplexer 1204 as shown in FIG. 19.

Another concatenated arrangement of another plurality of laser cavities(not shown but analogous to ELCs 1-M forming the top superchannel) formsanother superchannel into the WDM multiplexer 1204, and the othersuperchannel is input into another input port of the WDM multiplexer1204 in FIG. 19.

The N×1 port coarse wavelength division multiplexing multiplexer 1204has a plurality of input ports (e.g., N), and each of the plurality ofinput ports is coupled to respective superchannels. The respectivesuperchannels are individually formed by concatenated arrangements ofdifferent plurality of laser cavities.

Each of the plurality of laser cavities 24 includes an optical gain chip10 attached to the semiconductor substrate 30 and the integratedphotonic circuit 25 on the semiconductor substrate 30, where the opticalgain chip is optically coupled to the integrated photonic circuitthereby forming a laser cavity 24. The integrated photonic circuitcomprises: the active intra-cavity thermo-optic optical phase tunerelement 230, the intra-cavity optical band-pass filter 120, and theoutput coupler band-reflect optical grating filter with passive phasecompensation 140.

The WDM multiplexer may be a course wavelength division multiplexingmultiplexer (which has been illustrated) and/or a dense wavelengthdivision multiplexing multiplexer. The microcontroller 1220 isconfigured to calibrate each of the plurality of laser cavities 24 (persuperchannel) to a WDM multiplexer pass band of the WDM multiplexer 1204according to the self-calibration routine 1400. During theself-calibration routine, the microcontroller 1220 is configured tocalibrate individual laser wavelengths of the plurality of lasercavities 24 to either a blue edge or a red edge of the WDM multiplexerpass band of the WDM multiplexer. The superchannel is formed by theindividual laser wavelengths of the plurality of laser cavities 24.After performing the self-calibration routine, the microcontroller 1220is configured to provide offsets between the individual laserwavelengths (i.e., to space the operating wavelengths) of the pluralityof laser cavities in the superchannel.

It will be noted that various semiconductor device fabrication methodsmay be utilized to fabricate the components/elements discussed herein asunderstood by one skilled in the art. In semiconductor devicefabrication, the various processing steps fall into four generalcategories: deposition, removal, patterning, and modification ofelectrical properties.

Deposition is any process that grows, coats, or otherwise transfers amaterial onto the wafer. Available technologies include physical vapordeposition (PVD), chemical vapor deposition (CVD), electrochemicaldeposition (ECD), molecular beam epitaxy (MBE) and more recently, atomiclayer deposition (ALD) among others.

Removal is any process that removes material from the wafer: examplesinclude etch processes (either wet or dry), and chemical-mechanicalplanarization (CMP), etc.

Patterning is the shaping or altering of deposited materials, and isgenerally referred to as lithography. For example, in conventionallithography, the wafer is coated with a chemical called a photoresist;then, a machine called a stepper focuses, aligns, and moves a mask,exposing select portions of the wafer below to short wavelength light;the exposed regions are washed away by a developer solution. Afteretching or other processing, the remaining photoresist is removed.Patterning also includes electron-beam lithography.

Modification of electrical properties may include doping, such as dopingtransistor sources and drains, generally by diffusion and/or by ionimplantation. These doping processes are followed by furnace annealingor by rapid thermal annealing (RTA). Annealing serves to activate theimplanted dopants.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

1. A semiconductor chip configured to form a superchannel, thesemiconductor chip comprising: a plurality of laser cavities including afirst laser cavity, a next laser cavity, through a last laser cavity; awavelength division multiplexing (WDM) multiplexer; a plurality ofmodulators including a first modulator, a next modulator, through a lastmodulator, each of the plurality of modulators having a direct input, anadd port, and an output; and a concatenated arrangement of the pluralityof laser cavities to form the superchannel, the concatenated arrangementincluding: the last laser cavity coupled to the direct input of the lastmodulator, and the output of the last modulator coupled to the add portof the next modulator; the next laser cavity coupled to the direct inputof the next modulator, and the output of the next modulator coupled tothe add port of the first modulator; and the first laser cavity coupledto the direct input of the first modulator, and the output of the firstmodulator coupled to one input of the WDM multiplexer, thus forming thesuperchannel being input into the one input of the WDM multiplexer. 2.The semiconductor chip of claim 1, wherein another concatenatedarrangement of another plurality of laser cavities forms anothersuperchannel, the another superchannel being input into another input ofthe WDM multiplexer.
 3. The semiconductor chip of claim 1, wherein theWDM multiplexer has a plurality of inputs, each of the plurality ofinputs being coupled to respective superchannels.
 4. The semiconductorchip of claim 3, wherein the respective superchannels are individuallyformed by concatenated arrangements of a different plurality of lasercavities.
 5. The semiconductor chip of claim 1, wherein each of theplurality of laser cavities includes: an optical gain chip attached to asemiconductor substrate; and an integrated photonic circuit on thesemiconductor substrate, the optical gain chip optically coupled to theintegrated photonic circuit thereby forming a laser cavity.
 6. Thesemiconductor chip of claim 5, wherein the integrated photonic circuitcomprises: an active intra-cavity thermo-optic optical phase tunerelement; an intra-cavity optical band-pass filter; and an output couplerband-reflect optical grating filter with passive phase compensation. 7.The semiconductor chip of claim 1, wherein the WDM multiplexer is acourse wavelength division multiplexing multiplexer or a densewavelength division multiplexing multiplexer.
 8. The semiconductor chipof claim 1, further comprising a microcontroller configured to calibratethe plurality of laser cavities to a WDM multiplexer pass band of theWDM multiplexer according to a self-calibration routine; wherein, duringthe self-calibration routine, the microcontroller is configured tocalibrate individual laser wavelengths of the plurality of lasercavities to either a blue edge or a red edge of the WDM multiplexer passband of the WDM multiplexer; and wherein the superchannel is formed bythe individual laser wavelengths respectively of the plurality of lasercavities.
 9. The semiconductor chip of claim 8, wherein, afterperforming the self-calibration routine, the microcontroller isconfigured to provide offsets between the individual laser wavelengthsof the plurality of laser cavities in the superchannel. 10-18.(canceled)
 19. A semiconductor chip configured as a receiver to receivea superchannel, the semiconductor chip comprising: a polarizationsplitter rotator configured to receive and split received light of thesuperchannel; wavelength division multiplexing (WDM) demultiplexersconfigured to demultiplex the received light; counter propagating dropfilters configured to capture the received light at a particular targetwavelength and generate an electrical signal; wherein each of thecounter propagating drop filters is coupled to an electrical receiver,the electrical receiver receives the electrical signal corresponding tothe particular target wavelength.
 20. The semiconductor chip of claim19, wherein the counter propagating drop filters comprise: a ringresonator configured to capture the received light at the particulartarget wavelength from a waveguide; and a photodiode that converts thereceived light at the particular wavelength into the electrical signalthat is sent to the electrical receiver.